test: var to const in tls-no-cert-required

PR-URL: https://github.com/nodejs/node/pull/9800
Reviewed-By: Roman Reiss <me@silverwind.io>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>

test/parallel/test-tls-no-cert-required.js

@@ -1,12 +1,12 @@
 'use strict';
-var assert = require('assert');
-var common = require('../common');
+const assert = require('assert');
+const common = require('../common');
 
 if (!common.hasCrypto) {
   common.skip('missing crypto');
   return;
 }
-var tls = require('tls');
+const tls = require('tls');
 
 // Omitting the cert or pfx option to tls.createServer() should not throw.
 // AECDH-NULL-SHA is a no-authentication/no-encryption cipher and hence