doc: add directory structure in writing-tests.md

PR-URL: https://github.com/nodejs/node/pull/18802
Fixes: https://github.com/nodejs/node/issues/18774


Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Matheus Marchini <matheus@sthima.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>
Reviewed-By: Shingo Inoue <leko.noor@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>