Add test suite

[jpoly1219]

Currently, users must manually test their implementations by interacting with the web UI. A test suite to test user implementations would be a nice touch.

Confirmed and requested by Dr. Omar.

[cyrus-]

Testing infrastructure now available tdue to #8, but still need to add a more thorough test suite covering all of the judgements in the paper. Tests should be written to avoid the common mistake students make of only handling the value forms in the subexpressions, e.g. testing Plus(NumLit(2), NumLit(2)) isn't as good as testing Plus(Plus(NumLit(2), NumLit(3)), Plus(NumLit(4), NumLit(5)).