test: fix flaky http-client-timeout-agent

Close the connection immediately after receiving the `end` event to
avoid that the `timeout` event is fired before the connection closes
gracefully.
Move the test back to parallel.

PR-URL: #19856
Fixes: #19804
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>

test/sequential/test-http-client-timeout-agent.js → test/parallel/test-http-client-timeout-agent.js

@@ -48,11 +48,10 @@ const server = http.createServer(function(req, res) {
 
 server.listen(0, options.host, function() {
   options.port = this.address().port;
-  let req;
 
   for (requests_sent = 0; requests_sent < 30; requests_sent += 1) {
     options.path = `/${requests_sent}`;
-    req = http.request(options);
+    const req = http.request(options);
     req.id = requests_sent;
     req.on('response', function(res) {
       res.on('data', function(data) {
@@ -61,6 +60,7 @@ server.listen(0, options.host, function() {
       res.on('end', function(data) {
         console.log(`res#${this.req.id} end`);
         requests_done += 1;
+        req.destroy();
       });
     });
     req.on('close', function() {