Bok-Choy CMS Video Tests

Bok-Choy tests for [video.feature](https://github.com/edx/edx-platform/blob/master/cms/djangoapps/contentstore/features/video.feature)

@auraz @polesye @jzoldak 
In this case we can add to additional methods `getXblocks` and `getXblocksTypes` and use them in test class to be consistent with bok-choy guidelines.
My idea to make this code easier to read and I don't want see testing logic in the page object.