Bug 892048 - [test-agent] The test agent hangs when 1 test run is asked but another is already running r=yurenju

* increase the file watcher poll delay to reduce the CPU consumption. Eventually
  we'll want to use a inotify-based mechanism instead of the polling mechanism.
* batch file watcher results so that we can cancel it when we have too many
  modifications at once, especially when riding git branches
* properly queue new batch requests when one request is already happening
* don't schedule already existing requests
* upgrade mocha so that it doesn't think iframes are leaking globals