Animation controller test - #88251

Fixes test/animation/animation_controller_test.dart in #85160

Problem:
The test 'animateTo can deal with duration == Duration.Zero' was failing with the following error:

Expected: <0>
Actual: <5>
Expected no animation.
package:test_api expect
package:flutter_test/src/widget_tester.dart 484:3 expect
test/animation/animation_controller_test.dart 511:5 main.

Following line finds 5 transientCallbacks scheduled, while expecting zero, if shuffled with seed 123:

expect(SchedulerBinding.instance!.transientCallbackCount, equals(0), reason: 'Expected no animation.');

This is caused by some other test leaving transientCallbacks scheduled.

The tests I have identified leaving transientCallbacks behind is:
1. 'calling repeat with reverse set to true makes the animation alternate 'between lowerBound and upperBound values on each repeat' (1 callback)
2. calling repeat with specified min and max values makes the animation alternate between min and max values on each repeat (1 callback)
3. 2 AnimationBehavior > AnimationBehavior.normal runs "faster" than AnimationBehavior.preserve (2 callbacks)
4. 1 Simulations run forward (1 callback)
5. 1 Simulations run forward even after a reverse run (1 callback)
6. 1 Repeating animation with reverse: true report as forward and reverse (1 callback)

Fix:
By disposing the AnimationController after each test, the transientCallbacks get cleaned up.
I chose to dispose all controllers in every test, to make sure there is no risk of leaks.

Alternate fixes:

• I tried disposing the controller variable for all tests in tearDown instead of inside each test, but that broke some other tests due to controllers being disposed twice.
• One could consider to only dispose the controllers confirmed to break the test 'animateTocan deal with…'.

I am happy for any feedback about the best solution.

Pre-launch Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• I read the [Tree Hygiene] wiki page, which explains my responsibilities.
• I read and followed the [Flutter Style Guide], including [Features we expect every widget to implement].
• I signed the [CLA].
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test-exempt.
• All existing and new tests are passing.