testing-framework

We need a page in the docs that tell users that to mock they should use from unittest import mock.

We should note that the mock.patch decorators don't currently work, and that we need to use the context managers instead.

Pytest seems like it might be better for us then unittest:

• it works better with mypy (because mypy understands real asserts)
• It's easier to selectively run tests (CrossHair has a lot of not-fast tests)

Right now we just use the standard C random library, but really it'd be nice if the brute force fuzzer could do consistent tests across platforms, for various purposes. We probably need our own PRNG, something like:

https://www-cs-faculty.stanford.edu/~knuth/news02.html#rng

or

https://github.com/cmcqueen/simplerandom

I'm not sure what's best. We'd like decentish quality, maybe, and d