test: fix cluster-worker-wait-server-close races

Wait for data to arrive from worker before doing a disconnect. Without
this, whether the disconnect arrives at the worker before the master
accepts and forwards the connection descriptor to the worker is a race.

Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: Johan Bergström <bugs@bergstroem.nu>
Reviewed-By: Rod Vagg <rod@vagg.org>
PR-URL: https://github.com/nodejs/io.js/pull/1953
Fixes: https://github.com/nodejs/io.js/issues/1933
Fixes: https://github.com/nodejs/io.js/pull/1400