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

Yeah, for Messages Lib we don't need ORIGIN, but for the Activity Caller it's used in test like:
```
  apps[MessagesActivityCaller.ORIGIN] = __dirname + '/apps/activitycaller';

  var client = marionette.client({
    apps: apps
  });

````