Bug 991442 - Add coverage threshold to test-agent

https://bugzilla.mozilla.org/show_bug.cgi?id=991442
ok, and also probably when tests are run with the watcher instead of the client.

but then, maybe it could be output by the server only? Since the client loop on the output anyway, it should have it, right ? Or do we need it because we want to make the run fail when coverage is too low?