test: remove 1 second delay from test

In test-cluster-worker-wait-server-close, remove unneeded 1-second delay
and refactor to eliminate flakiness on FreeBSD.

PR-URL: https://github.com/nodejs/node/pull/4616


Reviewed-By: Johan Bergström <bugs@bergstroem.nu>
Reviewed-By: James M Snell <jasnell@gmail.com>