Bug 1089464 - [Smart Screen][Home]Launch card directly from smart home

To be able launch app from Smart Home directly, with some more unit tests also.

https://bugzilla.mozilla.org/show_bug.cgi?id=1089464
This is required because the chain is just a little bit too long... But I don't have good solution either.
What I can come up is:
1. Just make a stub of `_loadFile` or `_loadDefaultCardList`
2. move setTimeout to `MockXMLHttpRequest.send` or etc.

But We can just leave it here since it's not our focus on test cases currently.