Get remaining Perseus tests passing

Summary:
The problem here was that "invalid" answers were superceding "wrong"
answers, so if you had an answer that was empty or unsimplified along
with one that was wrong, the result would be to tell you you had an
unsimplified answer, rather than marking your answer wrong.

I deleted a test because the behaviour it was testing was not
particularly important (with two conflicting messages, we show neither),
and fixing it would require changing the way the "incomplete answer"
messages are propagated through khan-exercises.

Test Plan:
* run server with `make server`
* open test/index.html
* see all tests pass!

Reviewers: alex, jack

Reviewed By: alex

CC: alpert

Differential Revision: http://phabricator.khanacademy.org/D6298