* Avoid notice by making `WP_Image_Editor_Mock::test()` compatible with `WP_Image_Editor::test()`.
* Suppress deprecated function notice for `wp_load_image()`.
* Add assertion for `wp_get_image_editor()`.

See #25282.




git-svn-id: https://develop.svn.wordpress.org/trunk@25380 602fd350-edb4-49c9-b593-d223f7449a82