test: update test-cluster-disconnect-unshared-udp

Changed var to const

PR-URL: https://github.com/nodejs/node/pull/8599
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Ilkka Myller <ilkka.myller@nodefield.com>

test/parallel/test-cluster-disconnect-unshared-udp.js

@@ -7,11 +7,11 @@ if (common.isWindows) {
   return;
 }
 
-var cluster = require('cluster');
-var dgram = require('dgram');
+const cluster = require('cluster');
+const dgram = require('dgram');
 
 if (cluster.isMaster) {
-  var unbound = cluster.fork().on('online', bind);
+  const unbound = cluster.fork().on('online', bind);
 
   function bind() {
     cluster.fork({BOUND: 'y'}).on('listening', disconnect);
@@ -23,7 +23,7 @@ if (cluster.isMaster) {
   }
 } else {
   if (process.env.BOUND === 'y') {
-    var source = dgram.createSocket('udp4');
+    const source = dgram.createSocket('udp4');
 
     source.bind(0);
   }