use pytest (bug 1090184)

This builds on top of https://github.com/mozilla/olympia/pull/366 (which got rid of `test_utils`), and adds pytest (which is awesome).

Fixes [bug 1090184](https://bugzilla.mozilla.org/show_bug.cgi?id=1090184)
Actually, you're right, it makes more sense and simplifies the code ;) Thanks!