[a11y] Improve EmbedderA11yTest, fixture comments

[cbracken]

Applies cleanup/simplification to EmbedderA11yTest unit tests.

• Add numbered breadcrumbs to identify corresponding sections of the C++ and Dart fixture tests.
• Eliminates duplicated native method implementations for notifying:
  • whether semantics is enabled
  • whether a particular accessibility feature is enabled
  • which semantics action was fired

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 and the C++, Objective-C, Java style guides.
• I listed at least one issue that this PR fixes in the description above.
• I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test-exempt. See testing the engine for instructions on writing and running engine tests.
• I updated/added relevant documentation (doc comments with ///).
• I signed the CLA.
• All existing and new tests are passing.

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[cbracken]

This is stacked on top of #40993 the relevant diffs are in the second commit in this PR.

[loic-sharma]

Thanks for doing this! 🏆

[cbracken]

Alright I've reverted back to just cleaning up the comments as it was originally. I'll move the other refactoring to another patch.