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/selectors/builder.rs

@@ -84,12 +84,6 @@ impl<Impl: SelectorImpl> SelectorBuilder<Impl> {
         self.current_len = 0;
     }
 
-    /// Returns true if no simple selectors have ever been pushed to this builder.
-    #[inline(always)]
-    pub fn is_empty(&self) -> bool {
-        self.simple_selectors.is_empty()
-    }
-
     /// Returns true if combinators have ever been pushed to this builder.
     #[inline(always)]
     pub fn has_combinators(&self) -> bool {