Tests: Add test class for wp_enqueue_stored_styles().
[54214] added the wp_enqueue_stored_styles() tests and clean_up_global_scope() reset global method to Tests_Theme_wpGetGlobalStylesheet.
This changeset relocates those tests a new test class specifically for wp_enqueue_stored_styles() and removes Tests_Theme_wpGetGlobalStylesheet::clean_up_global_scope() method.
Why not relocate the clean_up_global_scope() method to the new test class?
The test class extends from WP_Theme_UnitTestCase which includes this method.
Follow-up to [54214], [54703].
Props costdev.
See #57841.