test: cleanup/update test-dgram-empty-packet.js

* Changed some `var` to `const` and 'let'
 * Changed `==` to `===` for clarity.

PR-URL: https://github.com/nodejs/node/pull/8896
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Ilkka Myller <ilkka.myller@nodefield.com>
Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

test/parallel/test-dgram-empty-packet.js

@@ -1,22 +1,22 @@
 'use strict';
-var common = require('../common');
+const common = require('../common');
+const dgram = require('dgram');
 
-var dgram = require('dgram');
+let callbacks = 0;
-var callbacks = 0;
+let timer;
-var client;
-var timer;
 
 if (common.isOSX) {
   common.skip('because of 17894467 Apple bug');
   return;
 }
 
-client = dgram.createSocket('udp4');
+const client = dgram.createSocket('udp4');
 
 client.bind(0, function() {
+
   function callback() {
     callbacks++;
-    if (callbacks == 2) {
+    if (callbacks === 2) {
       clearTimeout(timer);
       client.close();
     } else if (callbacks > 2) {