Tests for conditional content (split_test module).

TNL-38 (TNL-39 is the unit test subtask).

@dan-f and @andy-armstrong please review
To be a little more specific - It's not clear to me which user is submitting an answer to the question. My assumption is that in the `setup` case, the staff user is defining an answer to the problem, and in the `test` case, the student is submitting an answer... is that correct?