tests: update test-tls-no-cert-required

Change var to const.
Wrap callback of createServer with common.mustCall.

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

@@ -1,15 +1,16 @@
 'use strict';
-var common = require('../common');
+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
 // doesn't need a certificate.
-tls.createServer({ ciphers: 'AECDH-NULL-SHA' }).listen(0, function() {
-  this.close();
-});
+tls.createServer({ ciphers: 'AECDH-NULL-SHA' })
+  .listen(0, common.mustCall(function() {
+    this.close();
+  }));