Ensure that every test answer belongs to the test-corresponding question. #4181, #4084.

This was done by reviewing all uses of Factory(:answer). This seems to
cover the bulk of the CI failures caused by this problem, but I
haven't gone through all 100+ of them to verify that it covers them
all.
Merge pull request #203 from NUBIC/4181-test_questions_answers

Ensure that every test answer belongs to the test-corresponding question. Closes #4181.