Verify grade export works for problems in split tests

Add tests to verify that grade export works (does not break) when units in graded subsections contain problems in split tests, which is achievable in studio.

TNL-41

@cahrens please review
OK, thanks. Please add the comment i"Verify that both students were successfully graded" in this case too. In fact, you could create a helper method to do lines 663-683 (pass in expected CSV rows) and remove the code duplication for the two cases (this is a nit).