REST API: Re-introduce WP_Test_REST_Widgets_Controller tests.
Returns the test suite with the addition of defining a URL in the RSS widget within WP_Test_REST_Widgets_Controller::test_get_items() to work around an infinite loop in WP_Widget_RSS on PHP 8 if the URL is unset or empty.
Reverts [51020].
Props peterwilsoncc, TimothyBlynJacobs.
See #41683.