Popups currently makes use of a render cache. Using a cache in this
way without protection causes problems (I will look at cleaning this up)
The hack below it
`mw.popups.$popup.html( mw.popups.$popup.html() );`
will lead to the html of the cached object being wiped out on IE9 meaning
future loads will show an empty popup.
Add a unit test (which previously would fail in IE9) to protect against this
in future.
Bug: T68496
Change-Id: Icef784fb389b0ab1856e2ba7464c9423ebcd62ab
