Add answer-checking unit tests!

I recently fixed some bugs in khan-exercises related to the intersection of multiple inputs and unsimplified answers. The same bugs exist in perseus. But before fixing them, I figured I'd write some unit tests. Turns out there are even more differences in how we grade between khan-exercises and perseus than I thought.

Test Plan: Run the unit tests! http://perseus.ka.local/test. Some of them fail...but just you wait!

Auditors: alex