Bug 1856018 - Make forgiving selectors serialize. r=dshin

As per resolution in https://github.com/w3c/csswg-drafts/issues/8356

Safari already does this. Expand the tests they tweaked to be useful
again (test that when non-forgiving parsing is used, then the selector
is invalid).

Differential Revision: https://phabricator.services.mozilla.com/D189666

servo/components/selectors/matching.rs

@@ -1100,6 +1100,7 @@ where
             );
             anchor.map_or(false, |a| a == element.opaque())
         },
+        Component::Invalid(..) => false,
     }
 }