Don't rerun tests if the code hasn't changed

[virtuald]

Via @ArthurAllshire:

Also, it would be nice if the library could detect that you had already run the tests on a particular code base using a checksum, and then not run them again when you are deploying, or trying to run them again.

This is a bit complex if you really want to do it right, since you should probably hash imports et al. But probably doable.

I won't have time to get to this anytime soon, but it's a good idea.