doc: update examples in writing-tests.md

`writing-tests.md` states to use arrow functions when appropriate.
This updates the examples to do that.

Further, this syncs the docs with what's found in
[`test/parallel/test-http-agent-null.js`](https://github.com/nodejs/node/blob/master/test/parallel/test-http-agent-null.js)

PR-URL: #30126
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>

doc/guides/writing-tests.md

@@ -162,19 +162,22 @@ const assert = require('assert');
 const http = require('http');
 
 let request = 0;
+let listening = 0;
 let response = 0;
-process.on('exit', function() {
+process.on('exit', () => {
   assert.equal(request, 1, 'http server "request" callback was not called');
+  assert.equal(listening, 1, 'http server "listening" callback was not called');
   assert.equal(response, 1, 'http request "response" callback was not called');
 });
 
 const server = http.createServer((req, res) => {
   request++;
   res.end();
-}).listen(0, function() {
+}).listen(0, () => {
+  listening++;
   const options = {
     agent: null,
-    port: this.address().port
+    port: server.address().port
   };
   http.get(options, (res) => {
     response++;
@@ -193,16 +196,16 @@ const http = require('http');
 
 const server = http.createServer(common.mustCall((req, res) => {
   res.end();
-})).listen(0, function() {
+})).listen(0, common.mustCall(() => {
   const options = {
     agent: null,
-    port: this.address().port
+    port: server.address().port
   };
   http.get(options, common.mustCall((res) => {
     res.resume();
     server.close();
   }));
-});
+}));
 
 ```
 
@@ -216,7 +219,7 @@ shutting down an HTTP server after a specific number of requests).
 ```javascript
 const Countdown = require('../common/countdown');
 
-const countdown = new Countdown(2, function() {
+const countdown = new Countdown(2, () => {
   console.log('.');
 });