Bug 1098515 - [Usage][Tests] Prepare the ground for MarionetteJS integration tests. r=salva

Yeah, I was thinking about that too and we have the same thing in SMS, but currently can't think of any good way to generalize it because of ```Components.utils``` code, eg. ```Cu.waiveXrays```, ```Cu.cloneInto``` that are used for all return values. But will give it a second thought anyway :)