Restore the test code for `get_theme()`, add the notice suppression filters, and fix the type in the `remove_filter()` call.

See #25282.



git-svn-id: https://develop.svn.wordpress.org/trunk@25362 602fd350-edb4-49c9-b593-d223f7449a82
Set `$_SERVER['REQUEST_METHOD'] = null` in `Tests_Auth::setUp()` to suppress undefined index notices when `wp_validate_auth_cookie()` is called.

See #25282.



git-svn-id: https://develop.svn.wordpress.org/trunk@25363 602fd350-edb4-49c9-b593-d223f7449a82