Fix answers not pairing up with similar scores.

The sort by answer score wasn't working cause I assigned it the score
model object instead of the actual score.

This was actually picked up by the very basic score matched answer
pairing test.
Add more tests for the judgements module.

Particularly, the answer pair generation part. Unfortunately, due to
time constraints, the score matched pairing test is very basic. It
definitely needs to be improved upon.