Bug 1053964 - [Messages][Tests] Add marionette JS test case to verify close\back button behaviour when app is in activity mode. r=julien

I was thinking about it, but I don't really like that it changes current frame and kind of one way (I've not found way to save current frame before switching and restoring it after check). IMO it should be somewhere on "upper level" than messages lib, but still don't have any strong opinion on that. Will think about it again.