Docgen: split getIntermediateRepresentation unit tests in smaller tests

[ciampo]

Description

The getIntermediateRepresentation unit tests currently have a lot of assertions per test, making them hard to read and debug in case of failure.
This PR aims at splitting the assertion into smaller tests.

The actual code for the tests hasn't been updated. The main changes are:

• Add one intermediate level of describe around most tests
• Split long tests into smaller ones

Most of the diff changes are caused by the change in indentation.

How has this been tested?

• Run npm run test-unit packages/docgen
• Make sure that all tests are passing
• Check how the list of tests for getIntermediateRepresentation is more granular

Screenshots

N/A

Types of changes

Refactor

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• (N/A) I've tested my changes with keyboard and screen readers.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.
• I've updated all React Native files affected by any refactorings/renamings in this PR (please manually search all *.native.js files for terms that need renaming or removal).

[github-actions]

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[ntsekouras]

Thanks for the contribution @ciampo 👍 !