Simplify playlist block edit tests

[scruffian]

What?

Refactors Playlist and Playlist Track unit tests to use real block-editor components and stores instead of mocks.

Why?

The previous tests mocked too much of the editor environment, which made them unrepresentative of how the block actually behaves.

How?

The tests now use real components, and use a small editor.MediaUpload filter only to provide deterministic media selections.

Testing Instructions

1. Run npm run test:unit packages/block-library/src/playlist/test/edit-component.js packages/block-library/src/playlist-track/test/edit.js -- --runInBand.
2. Run npm run lint:js -- packages/block-library/src/playlist/test/edit-component.js packages/block-library/src/playlist-track/test/edit.js.

Testing Instructions for Keyboard

Not applicable; this PR only changes test code.

Screenshots or screencast

Not applicable.

Use of AI Tools

AI assistance was used to inspect the existing tests, refactor them, and run validation; the resulting changes were reviewed before submission.

[github-actions]

Size Change: 0 B

Total Size: 7.73 MB

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[github-actions]

Flaky tests detected in cfca7bd.
Some tests passed with failed attempts. The failures may not be related to this commit but are still reported for visibility. See the documentation for more information.

🔍 Workflow run URL: https://github.com/WordPress/gutenberg/actions/runs/32055498135
📝 Reported tests:

should allow user to add and remove multiple local font files in /test/e2e/specs/admin/font-library.spec.js, passed after 1 failed attempt.

Error: expect(locator).toBeVisible() failed

Locator: getByLabel('Exo 2 Semi-bold Italic')
Expected: visible
Timeout: 5000ms
Error: element(s) not found

Call log:
  - Expect "toBeVisible" with timeout 5000ms
  - waiting for getByLabel('Exo 2 Semi-bold Italic')

    at /home/runner/work/gutenberg/gutenberg/test/e2e/specs/admin/font-library.spec.js:58:6

[github-actions]

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