test: refactor test-http-pause-resume-one-end

* setTimeout() with no duration -> setImmediate()
* var -> const
* remove unused variable `chunk`

PR-URL: nodejs#10210
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>

test/parallel/test-http-pause-resume-one-end.js

@@ -1,23 +1,23 @@
 'use strict';
 const common = require('../common');
-var http = require('http');
+const http = require('http');
 
-var server = http.Server(function(req, res) {
+const server = http.Server(function(req, res) {
   res.writeHead(200, {'Content-Type': 'text/plain'});
   res.end('Hello World\n');
   server.close();
 });
 
 server.listen(0, common.mustCall(function() {
-  var opts = {
+  const opts = {
     port: this.address().port,
     headers: { connection: 'close' }
   };
 
   http.get(opts, common.mustCall(function(res) {
-    res.on('data', common.mustCall(function(chunk) {
+    res.on('data', common.mustCall(function() {
       res.pause();
-      setTimeout(function() {
+      setImmediate(function() {
         res.resume();
       });
     }));