[Flutter GPU] Establish a better unittesting scheme to cover API behavior changes.

[bdero]

So far, we have:

• Impeller "Dart renderer unittests": renderer_dart_unittests.cc + dart_tests.dart
  • Runs in CI.
  • Been using this primarily to cover the API in a crash smoke test.
  • Can't actually draw anything visual to the playgrounds.
• flutter_tester: gpu_test.dart
  • Runs in CI.
  • Up until now, I've only used this to test what happens when someone tries to use Flutter GPU even though Impeller isn't available. Now that flutter_tester supports Impeller, this is potentially a good way to perform behavioral testing and perhaps submit goldens.

We should clean up this testing situation and establish a clear pattern. My immediate suggestion would be:

1. See if we can do all behavioral testing/golden collection using flutter_tester at this point.
2. Stretch goal: Make the "Dart renderer unittests" actually display to Impeller's playground. This would be a convenient/useful debugging tool.

[bdero]

@chinmaygarde is going to fix up the playground situation so that we can render to textures.