Composite: add legacy unit tests to stable version

[ciampo]

What?

Follow-up to #65720 (comment)

This PR adds the same suite of unit tests that currently runs against the legacy version of Composite to the stable version too.

Why?

Testing gives us confidence and helps us prevent regressions.

How?

• Copied the suite of unit tests applied to legacy Composite
• Made a few tweaks:
  • Removed most abstractions in favour of test readability;
  • Changes item selectors from getByText to getByRole
  • adapted the tests to the stable Composite component's APIs

Testing Instructions

• Code review
• Make sure unit tests are passing

No runtime changes

[github-actions]

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[github-actions]

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Co-authored-by: ciampo <mciampini@git.wordpress.org>
Co-authored-by: mirka <0mirka00@git.wordpress.org>

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[mirka]

Looks good 👍