Moves test ownership validation to flutter/flutter

[gaaclarke]

issue: #188640

This follows the pattern from #161249

This duplicates the logic of https://github.com/flutter/cocoon/blob/main/app_dart/bin/validate_task_ownership.dart

Note that the TESTOWNERS file format seems straightforward but it is actually weird where things that look like comments have semantic meaning.

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the AI contribution guidelines and understand my responsibilities, or I am not using AI tools.
• 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 this PR is test-exempt.
• I followed the breaking change policy and added Data Driven Fixes where supported.
• All existing and new tests are passing.

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Code Review

This pull request removes the Linux test_ownership target from .ci.yaml and introduces a new Dart test, test_ownership_validation_test.dart, to validate that all CI targets in .ci.yaml have a defined owner in TESTOWNERS. Feedback on the changes highlights a compile-time error caused by attempting to instantiate the abstract class YamlMap directly, and suggests using RegExp(r'\s+') instead of split(' ') to robustly handle multiple consecutive spaces when parsing target names and owners.

[gaaclarke]

I verified it was hooked up to ci, when introducing a failure the following tests failed.

[jtmcdole]

Unpopular opinion, but I like:

  final isShard = switch ((recipe, tags?.contains('shard'), properties)) {
    ('flutter/flutter_drone', _, _) => true,
    (_, true, _) => true,
    (_, _, final p) when p['shard'] != null => true,
    _ => false,
  };

[gaaclarke]

Oh yea, that's nice. I don't usually create new structures in order to send them to a match since I'm never sure if they are optimized away or not. It could be generating extra garbage.