test: fix race condition in test-http-client-onerror

Occasionally test-http-client-onerror will fail with a refused connection.
This patch fixes the possibility that connections will be attempted before
server is listening.

PR-URL: https://github.com/nodejs/node/pull/4346


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: Johan Bergström <bugs@bergstroem.nu>
Reviewed-By: James M Snell <jasnell@gmail.com>