Making sure we're testing test_data/**/test

[HosseinYousefi]

For example we seem not to be testing https://github.com/dart-lang/native/blob/main/pkgs/native_assets_builder/test_data/treeshaking_native_libs/test/treeshaking_native_libs_test.dart

As seen here: https://github.com/dart-lang/sdk/blob/main/pkg/dartdev/test/native_assets/test_test.dart

What if we have a test_runner.yaml file that needs to exist for every test_data that has a test/ directory. This is verifiable through CI. The logic in the sdk test could iterate through test_runner.yaml files and verify that everything works correctly.

@dcharkes @mosuem

[mosuem]

Do we want to run those tests? I thought we didn't. I use them for manual testing during development only.