test: refacor spawn[Sync]Pwd

* extract the gist into common.pwdCommand
* Merge test-child-process-buffering.js into test-child-process-stdio.js

PR-URL: https://github.com/nodejs/node/pull/22522


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>