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.
Other apps prefer to put the inclusion of mocks_helper at setup.js, like: https://github.com/mozilla-b2g/gaia/blob/master/apps/settings/test/unit/setup.js 

So, it is fine to put here. But I prefer to create a setup.js file and move the line there.