test: improve test-cluster-net-listen.js

convert var to const

PR-URL: nodejs#9953
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>

test/parallel/test-cluster-net-listen.js

@@ -1,8 +1,8 @@
 'use strict';
-var common = require('../common');
-var assert = require('assert');
-var cluster = require('cluster');
-var net = require('net');
+const common = require('../common');
+const assert = require('assert');
+const cluster = require('cluster');
+const net = require('net');
 
 if (cluster.isMaster) {
   // ensure that the worker exits peacefully