Added a (failing) test for HTMLScriptElement.innerHTML

innerHTML handling for Script elements should use a different escape mechanism (or none?) from the rest of the elements.

This test currently fails because the innerHTML string for the script element is escaped to be '&lt;x>{{y}}&lt;/x>'. 

I encountered this issue when investigating why some Angular (Dart) tests were failing on ShadowDOM. Angular allows HTML templates from script elements.