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)
It seemed useless. I thought about using `csv.DictReader`, but it needs to read from a file-like object (which is ok for `response.content`, but I had to trick it with `StringIO` or something for the "expected" string).

Do you think I still should do that?