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
`assertDictContainsSubset` asserts that its first argument is a subset of its second - for example `assertDictContainsSubset({'foo': 'bar'}, {'foo': 'bar', 'baz': 'qux'})` would pass, while `assertDictContainsSubset({'foo': 'bar'}, {'baz': 'qux'})` would fail.

Now, `result` here is the return value of the task helper function which generates CSV data from the students' grades. All the functions in the `tasks_helper` module return dicts representing the work they did. So we're just verifying that `upload_grades_csv` actually succeeded grading the two students in this course.