[selectors] Add new test for :focus-visible

This test checks that when you click an element to focus it,
it doesn't show any kind of focus ring. See issue #4278.

Currently Firefox passes this test, by Chromium fails it
because Chromium is using `:focus` on the default UA stylesheet
and is adding an outline on the element, despite it doesn't match
:focus-visible (see https://crbug.com/1162070).

css/selectors/focus-visible-001.html

@@ -11,19 +11,13 @@
   <script src="/resources/testdriver-actions.js"></script>
   <script src="/resources/testdriver-vendor.js"></script>
   <style>
-    @supports not (selector(:focus-visible)) {
-      :focus {
-        background-color: tomato;
-      }
-    }
-
     :focus-visible {
-      outline: darkgreen solid 5px;
+      outline: green solid 5px;
     }
 
     :focus:not(:focus-visible) {
       outline: 0;
-      background-color: tomato;
+      background-color: red;
     }
   </style>
 </head>
@@ -39,7 +33,8 @@
   <script>
     async_test(function(t) {
       el.addEventListener("focus", t.step_func(function() {
-        assert_equals(getComputedStyle(el).outlineColor, "rgb(0, 100, 0)");
+        assert_equals(getComputedStyle(el).outlineColor, "rgb(0, 128, 0)", `outlineColor for ${el.tagName}#${el.id} should be green`);
+        assert_not_equals(getComputedStyle(el).backgroundColor, "rgb(255, 0, 0)", `backgroundColor for ${el.tagName}#${el.id} should NOT be red`);
         t.done();
       }));
       const tab_key = '\ue004';