test: update to const iin cluster test

Update `var` to `const`

PR-URL: nodejs#10007
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

test/parallel/test-cluster-shared-handle-bind-privileged-port.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 (common.isWindows) {
   common.skip('not reliable on Windows');
@@ -21,7 +21,7 @@ if (cluster.isMaster) {
     assert.strictEqual(exitCode, 0);
   }));
 } else {
-  var s = net.createServer(common.fail);
+  const s = net.createServer(common.fail);
   s.listen(42, common.fail.bind(null, 'listen should have failed'));
   s.on('error', common.mustCall(function(err) {
     assert.strictEqual(err.code, 'EACCES');