testcases for RecommenderXBlock

I messed things up in the previous PR, and thus I make a new one. I think I should also change cms/djangoapps/contentstore/views/component.py , but I forget how to do it. Do you know how to do that, and could you help me check is there any other thing wrong, @pmitros ?
Oh, my mistake -- I thought you were using the `@data` to indicate `once`, `twice`, `thrice`, etc. Looking closer at the body of this test, it is indeed voting three times. This is fine.