Update Global Styles Controller PHPUnit tests

[Mamaduka]

What?

PR updates Global Styles Controller PHPUnit tests to match the core and reduce noise when running them.

How?

• Use @doesNotPerformAssertions instead of $this->markTestIncomplete(); when controlled doesn't implement the method.
• Removed find_and_normalize_global_styles_by_id helper method.
• Added test_get_item_schema tests.

Testing Instructions

CI is green.

Manual

npm run test:unit:php -- --filter Gutenberg_REST_Global_Styles_Controller_Test

Screenshots

Before

After

[github-actions]

Flaky tests detected in 26d84a4.
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/4041665345
📝 Reported issues:

• [Flaky Test] should select and deselect with shift and arrow keys #44382 in specs/editor/various/multi-block-selection.test.js