I understand. At this point in Ember’s life, I would probably still go ahead and load the whole app, and then test the models individually with stubbed responses via the method I mentioned above. I know it seems like overkill, but it beats spending time digging through framework code to figure out what you need to stub.
It looks like someone else on this list has been able to test their models in isolation (see Testing Ember Apps: how do you do it? - #2 by tchak). I haven’t tried it myself, but if there’s one thing I’ve learned from doing a lot of testing with Rails, it’s that stubbing out a framework is rarely the answer.