chore: add className testing util

[francinelucca]

Closes https://github.com/github/primer/issues/5255

This pull request adds a reusable test utility to verify that components correctly handle className props and applies it to the components' tests. The main focus is on improving test coverage and consistency for CSS class name behavior.

Changelog

New

• Added the implementsClassName function to utils/testing.tsx, which provides a standardized way to test that components render both their default and custom className props.
• Integrated implementsClassName into existing components test suite, ensuring that the components correctly apply both the base and custom class names.
• Adds CI to enforce the testing util is used in every component test file

Rollout strategy

• [ ] Patch release
• [ ] Minor release
• [ ] Major release; if selected, include a written rollout or migration plan
• [x] None; if selected, include a brief description as to why Test-only update

Testing & Reviewing

Merge checklist

• [x] Added/updated tests
• [ ] Added/updated documentation
• [ ] Added/updated previews (Storybook)
• [ ] Changes are SSR compatible
• [ ] Tested in Chrome
• [ ] Tested in Firefox
• [ ] Tested in Safari
• [ ] Tested in Edge
• [ ] (GitHub staff only) Integration tests pass at github/github (Learn more about how to run integration tests)

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Latest commit: 4ff3cee8c2bb26e468a5f58a64eb5f297435a2de

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🟢 ci completed with status success.

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