test: make listen-fd-cluster/server more robust

- eliminate unnecessary intermediate process ("parent")
- children exit if runner dies unexpectedly (killed on a test timeout,
  for example)
- use explicit messaging from children to parents to indicate when
  worker is ready to accept http requests, rather than racing to see
  whether runner will make request before worker is listening

PR-URL: https://github.com/nodejs/io.js/pull/1944


Reviewed-By: Johan Bergstrom <bugs@bergstroem.nu>
Reviewed-By: Ben Noordhuis <ben@strongloop.com>