Fixes part of #85160. This comes after #123751 as part of removing no-shuffle tags from all tests in the framework.

For render objects with the same depth, [paint] gets called in the order that [markNeedsPaint] was called, which is not necessarily the visual paint order. So having a test check the order of [paint] calls is brittle.

Here, it was breaking when an editable.foregroundPainter = null, intended to clean up a previous test, caused a [markNeedsPaint] on the composite foreground painter before the test could cause a [markNeedsPaint] on the background.

Instead, follow the pattern of the neighboring tests and use the paints matcher; remove the paintHistory static field.

For debugging this it was useful to have a toString on this test painter class, so add that too.

To be confident there weren't any remaining ordering-related flakes, I ran this test file with both the previously-failing seed in the comment and 308 other seeds, all of which passed. There are only 45 tests in this file, so an ordering-dependent flake would probably have a rate of at least 1/45 = 2.22% (the probability of a given test coming first, or of coming immediately after another given test), and the data is enough to reject that at p<.001, as well as to reject a 1% flake rate at p<.05.

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• I added new tests to check the change I am making, or this PR is test-exempt.
• All existing and new tests are passing.

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