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)
The real less compiler returns the name of the resulting file (which will then be added inline in the html). With `ls`, we also have the name of a file (the less file), which will thus be added inline in the html. We're lucky that the tests are actually grepping on a css rule that doesn't change once compiled!