style: Cleanup selector-matching for nested pseudo-elements, match ::slotted correctly when there's no selector before it, and add tests.

D29542 fixed the bogus checks that was making nested pseudo-elements match
author rules. This adds tests and ends up being just a cleanup, though as it
turns out we it also fixes an issue with ::slotted() matched from
Element.matches.

Differential Revision: https://phabricator.services.mozilla.com/D27529

components/style/invalidation/element/element_wrapper.rs

@@ -366,6 +366,10 @@ where
         self.element.is_root()
     }
 
+    fn is_pseudo_element(&self) -> bool {
+        self.element.is_pseudo_element()
+    }
+
     fn pseudo_element_originating_element(&self) -> Option<Self> {
         self.element
             .pseudo_element_originating_element()