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

I suppose here we could point to the unit test mocks. If your concern are the dates, we could use the same [_reviver_](https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Objects/JSON/parse) as used in [Cost Control Config](https://github.com/mozilla-b2g/gaia/blob/master/apps/costcontrol/js/config/config_manager.js#L146) or simply get rid of that representation once and for all.