Tests: Adjust Unicode tests for consistency.

Includes:
* Adding missing `@covers` tags.
* Correcting test class names as per the naming conventions.
* Moving `wp_check_invalid_utf8()` tests to their own file, separate from `wp_scrub_utf8()`.

Follow-up to [60630], [60793], [61000].

See #64225.

git-svn-id: https://develop.svn.wordpress.org/trunk@62207 602fd350-edb4-49c9-b593-d223f7449a82