Bug 991442 - Add coverage threshold to test-agent

https://bugzilla.mozilla.org/show_bug.cgi?id=991442
it won't work if we're in another directory name than "gaia" :/

can we "just" take the directory name where the json file is? That would mean the last part.
Except... we need to account for apps with entry points. So maybe we need to find manifest.webapp and take the directory where there is manifest.webapp + directory where there is metadata.json (if different).

Now, I don't think this should be in _this_ file in the test-agent. The test-agent is quite Gaia-agnostic actually. It takes js files, runs them, returns the result. I think we should try to do the same here. That means that all the application-specific things should actually be out of the test-agent it self, or at least in a separate file that we could "use" separately in the launcher (agent.js) in Gaia.

What do you think?