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/dom.rs

@@ -779,7 +779,7 @@ pub trait TElement:
     /// element-backed pseudo-element, in which case we return the originating
     /// element.
     fn rule_hash_target(&self) -> Self {
-        if self.implemented_pseudo_element().is_some() {
+        if self.is_pseudo_element() {
             self.pseudo_element_originating_element()
                 .expect("Trying to collect rules for a detached pseudo-element")
         } else {