test: always suffix `tmpdir`

This makes temp dir names consistent whether we run in stand-alone mode,
via `test.py` in single process, or in multi-process.

PR-URL: https://github.com/nodejs/node/pull/28035


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>