Scripts: Improve the way test files are discovered

[gziolo]

Description

Supersedes / closes #16263 opened by @Shelob9.

Test files matching has changed to fix the overlap between two types of tests implemented with test-e2e and test-unit.

This is a breaking change as we no longer match test files with .spec.js extension in the @wordpress/jest-preset-default. This extension is now used exclusively with e2e tests. This way we won't have to override test matching rules when using both types of tests, see WordPress core as an example:
https://github.com/WordPress/wordpress-develop/blob/94f4dcd15a8d85782323e1a87d15b172eebbf88e/tests/e2e/jest.config.js#L9-L11

wp-scripts test-e2e

Jest will look for test files with any of the following popular naming conventions:

• Files with .js (or .ts) suffix at any level of depth in spec folders.
• Files with .spec.js (or .spec.ts) suffix.

wp-scripts test-unit

Jest will look for test files with any of the following popular naming conventions:

• Files with .js (or .ts) suffix located at any level of depth in __tests__ folders.
• Files with .js (or .ts) suffix directly located in test folders.
• Files with .test.js (or .test.ts) suffix.

How has this been tested?

• npm run test-e2e
• npm run test-unit

[gziolo]

This branch:

Test Suites: 323 passed, 323 total
Tests:       3176 passed, 3176 total
Snapshots:   171 passed, 171 total
Time:        163.798s
Ran all test suites.

Master:

Test Suites: 328 passed, 328 total
Tests:       3320 passed, 3320 total
Snapshots:   175 passed, 175 total
Time:        145.631s, estimated 146s

It looks like 5 test files are skipped for some reasons.

[gziolo]

It should be fixed with e11195c. I had to rename all test files located in the test/integration folder.