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: #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>

Author: santigimeno

test/parallel/test-cluster-worker-isdead.js

@@ -2,7 +2,6 @@
 require('../common');
 var cluster = require('cluster');
 var assert = require('assert');
-var net = require('net');
 
 if (cluster.isMaster) {
   var worker = cluster.fork();
@@ -11,7 +10,7 @@ if (cluster.isMaster) {
             'created.');
 
   worker.on('exit', function() {
-    assert.ok(!worker.isConnected(),
+    assert.ok(worker.isDead(),
               'After an event has been emitted, ' +
               'isDead should return true');
   });