Fixes a bug where the UI wasn't updated after deleting/updating a theme.
Also introduces unit tests for theme management ajax handlers. For now they're
focused on `wp_ajax_update_theme()` but they can include tests for other
handlers as well.
Props chrisjean for initial patch.
Fixes#37924.
git-svn-id: https://develop.svn.wordpress.org/trunk@38710 602fd350-edb4-49c9-b593-d223f7449a82
