fix(tests): normalize scrollbars in headful tests (#2622)

Scrollbars look different on different platforms, so must be made invisible in tests. As a drive-by, xdescribe was broken with the new test runner.

References #2524

test/assets/frames/nested-frames.html

@@ -7,6 +7,9 @@ body iframe {
     flex-grow: 1;
     flex-shrink: 1;
 }
+::-webkit-scrollbar{
+    display: none;
+}
 </style>
 <script>
 async function attachFrame(frameId, url) {