Fix broken DOM tests

These were broken due to some changes in
d670020, which makes HUD.init require
that the document is loaded.

tests/dom_tests/dom_test_setup.js

@@ -1,11 +1,12 @@
 window.vimiumDomTestsAreRunning = true
 
 // Install frontend event handlers.
-HUD.init()
 Frame.registerFrameId({chromeFrameId: 0});
 
 getSelection = () =>
   window.getSelection().toString()
 
 // Shoulda.js doesn't support async code, so we try not to use any.
 Utils.nextTick = (func) => func()
+
+document.addEventListener("DOMContentLoaded", () => HUD.init());