Rewrite + speed up unit test script.

Summary: * Python is easier to maintain
* I want to add coverage support, so this rewrite makes it easier
* We run tests in parallel now, with a 3-4x speedup!
* Easy to run tests independently of fbmake (and avoid its slow startup time) -- and we can either test a single file or an entire directory @elgenie said this would be useful)

Reviewed By: @elgenie

Differential Revision: D1546618