Blocks: Add unit test coverage for Patterns registry.

Prior to this changeset, we did not seem to have any unit test coverage for the Patterns registry (`WP_Block_Patterns_Registry`).

With Block Hooks logic recently added ([56649]), it is particularly advisable to add some unit tests to the `get_registered()` and `get_all_registered()` methods to guard hooked block insertion against regressions.

Fixes #59476.