Add support for Firefox JS testing

This runs in it's current state (make sure you have slimerjs and casperjs installed), but quite a few tests are failing.  I don't think it feasible to get the tests working for 2.0 final, but it's something to keep in mind for a later release.
Oh, I see. That's a pain. The issue is that the caller normally decides whether or not to display the full output from the tests - if you do `iptest -j4`, you see a summary view, and it only shows the output from test groups that failed. I'll work out what we can do about it and make a PR against this branch.