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: https://github.com/nodejs/node/pull/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>