Try to fix flakiness in animation tests

Update all animations before forcing a reflow.

<!-- Please describe your changes on the following line: -->

---
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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [ ] These changes fix #___ (GitHub issue number if applicable)

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- [x] There are tests for these changes

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Try to fix flakiness in animation tests

Update all animations before forcing a reflow.

<!-- Please describe your changes on the following line: -->

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `___` with appropriate data: -->
- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [ ] These changes fix #___ (GitHub issue number if applicable)

<!-- Either: -->
- [x] There are tests for these changes

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<!-- Pull requests that do not address these steps are welcome, but they will require additional verification as part of the review process. -->

Try to fix flakiness in animation tests

Update all animations before forcing a reflow.

<!-- Please describe your changes on the following line: -->

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `___` with appropriate data: -->
- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [ ] These changes fix #___ (GitHub issue number if applicable)

<!-- Either: -->
- [x] There are tests for these changes

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Try to fix flakiness in animation tests

Update all animations before forcing a reflow.

<!-- Please describe your changes on the following line: -->

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `___` with appropriate data: -->
- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [ ] These changes fix #___ (GitHub issue number if applicable)

<!-- Either: -->
- [x] There are tests for these changes

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Try to fix flakiness in animation tests

Update all animations before forcing a reflow.

<!-- Please describe your changes on the following line: -->

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `___` with appropriate data: -->
- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [ ] These changes fix #___ (GitHub issue number if applicable)

<!-- Either: -->
- [x] There are tests for these changes

<!-- Also, please make sure that "Allow edits from maintainers" checkbox is checked, so that we can help you if you get stuck somewhere along the way.-->

<!-- Pull requests that do not address these steps are welcome, but they will require additional verification as part of the review process. -->

Try to fix flakiness in animation tests

Update all animations before forcing a reflow.

<!-- Please describe your changes on the following line: -->

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `___` with appropriate data: -->
- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [ ] These changes fix #___ (GitHub issue number if applicable)

<!-- Either: -->
- [x] There are tests for these changes

<!-- Also, please make sure that "Allow edits from maintainers" checkbox is checked, so that we can help you if you get stuck somewhere along the way.-->

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Fix flakiness in animation tests

1. When updating the animation timeline, ensure that nodes that are
    animating are marked dirty, if necessary, so any style queries will
    force a layout flush.
2. Disable the problematic transition test suites, as they are in Gecko.
    These suites often fail when Servo is so overloaded that it cannot
    deliver frames fast enough to get more than two samples during the
    animation lifecycle.

Fixes #28334.
Fixes #26435.
Fixes #21486.

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `___` with appropriate data: -->
- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix #28334, #26435, #21486.

<!-- Either: -->
- [x] There are tests for these changes

<!-- Also, please make sure that "Allow edits from maintainers" checkbox is checked, so that we can help you if you get stuck somewhere along the way.-->

<!-- Pull requests that do not address these steps are welcome, but they will require additional verification as part of the review process. -->

Fix flakiness in animation tests

1. When updating the animation timeline, ensure that nodes that are
    animating are marked dirty, if necessary, so any style queries will
    force a layout flush.
2. Disable the problematic transition test suites, as they are in Gecko.
    These suites often fail when Servo is so overloaded that it cannot
    deliver frames fast enough to get more than two samples during the
    animation lifecycle.

Fixes #28334.
Fixes #26435.
Fixes #21486.

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `___` with appropriate data: -->
- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix #28334, #26435, #21486.

<!-- Either: -->
- [x] There are tests for these changes

<!-- Also, please make sure that "Allow edits from maintainers" checkbox is checked, so that we can help you if you get stuck somewhere along the way.-->

<!-- Pull requests that do not address these steps are welcome, but they will require additional verification as part of the review process. -->

Fix flakiness in animation tests

1. When updating the animation timeline, ensure that nodes that are
    animating are marked dirty, if necessary, so any style queries will
    force a layout flush.
2. Disable the problematic transition test suites, as they are in Gecko.
    These suites often fail when Servo is so overloaded that it cannot
    deliver frames fast enough to get more than two samples during the
    animation lifecycle.

Fixes #28334.
Fixes #26435.
Fixes #21486.

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `___` with appropriate data: -->
- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix #28334, #26435, #21486.

<!-- Either: -->
- [x] There are tests for these changes

<!-- Also, please make sure that "Allow edits from maintainers" checkbox is checked, so that we can help you if you get stuck somewhere along the way.-->

<!-- Pull requests that do not address these steps are welcome, but they will require additional verification as part of the review process. -->

Fix flakiness in animation tests

1. When updating the animation timeline, ensure that nodes that are
    animating are marked dirty, if necessary, so any style queries will
    force a layout flush.
2. Disable the problematic transition test suites, as they are in Gecko.
    These suites often fail when Servo is so overloaded that it cannot
    deliver frames fast enough to get more than two samples during the
    animation lifecycle.

Fixes #28334.
Fixes #26435.
Fixes #21486.

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `___` with appropriate data: -->
- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix #28334, #26435, #21486.

<!-- Either: -->
- [x] There are tests for these changes

<!-- Also, please make sure that "Allow edits from maintainers" checkbox is checked, so that we can help you if you get stuck somewhere along the way.-->

<!-- Pull requests that do not address these steps are welcome, but they will require additional verification as part of the review process. -->