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 ?
You may want to make this method wrap [`self.assert_request_status_code()`](https://github.com/edx/edx-platform/blob/master/lms/djangoapps/courseware/tests/helpers.py#L42-L53), which is an assertion method that [I added to the codebase recently](https://github.com/edx/edx-platform/pull/4654) to replace the `check_for_get_code` and `check_for_post_code` functions. The code that you have works fine, but if you make it wrap `self.assert_request_status_code()`, it will be even shorter.