test: Update test-http-client-agent to use countdown timer

Fixes: nodejs#17169

test/parallel/test-http-client-agent.js

@@ -23,10 +23,10 @@
 const common = require('../common');
 const assert = require('assert');
 const http = require('http');
+const Countdown = require('../common/countdown');
 
 let name;
 const max = 3;
-let count = 0;
 
 const server = http.Server(common.mustCall((req, res) => {
   if (req.url === '/0') {
@@ -45,21 +45,23 @@ server.listen(0, common.mustCall(() => {
     request(i);
 }));
 
+const countdown = new Countdown(max, () => {
+  assert(!http.globalAgent.sockets.hasOwnProperty(name));
+  assert(!http.globalAgent.requests.hasOwnProperty(name));
+  server.close();
+});
+
 function request(i) {
   const req = http.get({
     port: server.address().port,
     path: `/${i}`
   }, function(res) {
     const socket = req.socket;
     socket.on('close', common.mustCall(() => {
-      ++count;
-      if (count < max) {
+      countdown.dec();
+      if (countdown.remaining > 0) {
         assert.strictEqual(http.globalAgent.sockets[name].includes(socket),
                            false);
-      } else {
-        assert(!http.globalAgent.sockets.hasOwnProperty(name));
-        assert(!http.globalAgent.requests.hasOwnProperty(name));
-        server.close();
       }
     }));
     res.resume();