test: fix test-cluster-worker-disconnect

- Just let the process exit gracefully after the worker is disconnected.

Reviewed-By: Brendan Ashworth <brendan.ashworth@me.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
PR-URL: https://github.com/nodejs/io.js/pull/1919
Fixes: https://github.com/nodejs/io.js/issues/1757