Bug 1028113 - FTU skip homescreen migration on tablet devices

You can't do this to verify that the message wasn't sent. Since you shouldn't call mTriggerLastRequestSuccess  (precisely because you're checking that no message was sent) mLastConnectionKeyword will be undefined, but I think it's better to check that no request was executed: 
assert.isNull(MockNavigatormozApps.mLastRequest);