test: favor const over in var for require in http test

Author: jun-oka

test/parallel/test-http.js

@@ -1,8 +1,8 @@
 'use strict';
 require('../common');
-var assert = require('assert');
-var http = require('http');
-var url = require('url');
+const assert = require('assert');
+const http = require('http');
+const url = require('url');
 
 var responses_sent = 0;
 var responses_recvd = 0;