test: fix cluster-worker-isdead

Check if the worker 'isDead' instead of 'isConnected' as the
'disconnect' event is not guaranteed to be received before the
'exit' event.
Remove the 'net' dependency as it is not used.

PR-URL: https://github.com/nodejs/node/pull/3954


Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Michaël Zasso <mic.besace@gmail.com>
Reviewed-By: Minwoo Jung <jmwsoft@gmail.com>