Add action/filter to `Tests_Theme::setUp()` to suppress errors while running back-compat tests on `get_theme()`, `get_themes()`, `get_theme_data()`, `get_current_theme()`.

See #25282.




git-svn-id: https://develop.svn.wordpress.org/trunk@25354 602fd350-edb4-49c9-b593-d223f7449a82