Allow tests to run concurrently.

Fsnotify (maybe inotify) generates multiple events on each file save.
These multiple events are dealt with by a timeout. However if the
first event runs tests which take longer than the timeout, the
subsequently generated events are not discarded but considered, and
the tests erroneously run again. This prevents such behavior by only
allowing the tests to run once at any given time, and as such multiple
incoming events are considered but since the tests are running discarded
immediately. (Maybe this will invert the situation and fast running
tests will be double-run :D)