Tests: Cover preservation of an existing admin parent file.
Verify that get_admin_page_parent() returns an empty string while preserving $parent_file when no submenu matches. Also cover skipping a matching submenu when the existing parent file contains a query string.
Follow-up to [64374].
Developed in: https://github.com/WordPress/wordpress-develop/pull/14312
Props huzaifaalmesbah.
See #65819.
Javascript required