test: use Countdown in test-http-set-cookies

PR-URL: #17504
Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Jon Moss <me@jonathanmoss.me>

test/parallel/test-http-set-cookies.js

@@ -23,9 +23,9 @@
 require('../common');
 const assert = require('assert');
 const http = require('http');
+const Countdown = require('../common/countdown');
 
-let nresponses = 0;
-
+const countdown = new Countdown(2, () => server.close());
 const server = http.createServer(function(req, res) {
   if (req.url === '/one') {
     res.writeHead(200, [['set-cookie', 'A'],
@@ -55,9 +55,7 @@ server.on('listening', function() {
     });
 
     res.on('end', function() {
-      if (++nresponses === 2) {
-        server.close();
-      }
+      countdown.dec();
     });
   });
 
@@ -72,14 +70,8 @@ server.on('listening', function() {
     });
 
     res.on('end', function() {
-      if (++nresponses === 2) {
-        server.close();
-      }
+      countdown.dec();
     });
   });
 
 });
-
-process.on('exit', function() {
-  assert.strictEqual(2, nresponses);
-});