test(chrome): compare the alias keys and the two derived facts

one page, both resolvers, the ten names that resolved on one host only.
each alias is asked beside the key it resolves through, so the pair is
pinned to the same elements rather than each to itself.

the page grows a container that overflows its box and a neighbour that
does not, because scrollable is derived from the box: without one the
only scrolling element on the page is the document root, whose answer
moves with the window.

internal/driver/chrome/testdata/selector-parity.html

@@ -43,6 +43,15 @@
         <option id="state_february">February</option>
       </select>
     </div>
+    <!-- scrollable is derived from the box rather than written by the markup,
+         and both producers state it only where it holds, so one container here
+         overflows its box and its neighbour does not. -->
+    <div id="scroll_row">
+      <div id="scroll_box" style="height: 20px; overflow: auto">
+        <div id="scroll_content" style="height: 200px">scrolls</div>
+      </div>
+      <div id="scroll_still">still</div>
+    </div>
     <script id="state_script">
       document.getElementById("state_remember").checked = true;
       document.getElementById("state_agree").checked = false;