Bug 1013464 -- intermittent dom.player is null during video_test.js unit testing

Load video.js after html is loaded into DOM to avoid video.js reference errors during unit testing runs. Also introduced more mocks to avoid errors during loading of video.js.
We should move mocksHelper, line 37~39 before `suite('Video App Unit Tests', function() {` because that doesn't load the ScreenLayout immediately and we don't need to delay the creation of mocksHelper.

And put line 41, `mocksHelper.init();`, after `suite('Video App Unit Tests', function() {` because we don't delay the loading the "ScreenLayout" file and the init function wires all mock object to window. So, we can use all mocked objects at suiteSetup, setup, and other tests. Or another choice is to put both of them after `suite('Video App Unit Tests', function() {`. That's fine to me