Reset flutter engine internal state in framework test postTest - #193842

…er#193842)

Part of flutter#192992

Follow-up to flutter#193497.

In flutter#193497, I introduced
`PlatformDispatcher.resetInternalState()` to allow test harnesses to
purge internal engine caches (such as Impeller's offscreen
`RenderTargetCache`) between test cases to ensure test isolation.

Now that the engine changes have rolled into the framework, this PR
connects the framework test harness:
1. Implements `TestPlatformDispatcher.resetInternalState()` in
`packages/flutter_test/lib/src/window.dart` to delegate to the
underlying `PlatformDispatcher`.
2. Invokes `platformDispatcher.resetInternalState()` in
`TestWidgetsFlutterBinding.postTest` alongside
`ServicesBinding.instance.resetInternalState()`.
3. Adds a unit test in
`packages/flutter_test/test/platform_dispatcher_test.dart` to verify
delegation.


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responsibilities.
- [x] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [x] I signed the [CLA].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [x] I updated/added relevant in-code documentation (doc comments with
`///`).
- [x] If this PR introduces a new feature or capability, I created and
linked a website documentation issue or PR in [flutter/website] (or
verified none is needed).
- [x] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [x] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [x] All existing and new tests are passing.

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