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
I'm surprised that this is exactly the same as in the previous test (in the previous, both student_a and student_b submitted answers, but here only student_a submitted an answer). I don't understand what assertDictContainsSubset is doing.