test: race condition in test-cluster-disconnect

The test was not waiting for all the worker-created sockets
to be listening before calling cluster.disconnect().
As a result, the channels with the workers could get closed
before all the socket handles had been passed to them, leading
to various errors.

test/simple/test-cluster-disconnect.js

@@ -35,6 +35,7 @@ if (cluster.isWorker) {
   }).listen(common.PORT + 1, '127.0.0.1');
 
 } else if (cluster.isMaster) {
+  var servers = 2;
 
   // test a single TCP server
   var testConnection = function(port, cb) {
@@ -52,7 +53,6 @@ if (cluster.isWorker) {
 
   // test both servers created in the cluster
   var testCluster = function(cb) {
-    var servers = 2;
     var done = 0;
 
     for (var i = 0, l = servers; i < l; i++) {
@@ -76,7 +76,7 @@ if (cluster.isWorker) {
       var worker = cluster.fork();
       worker.on('listening', function() {
         online += 1;
-        if (online === workers) {
+        if (online === workers * servers) {
           cb();
         }
       });