test: test validity of prefix in mkdtempSync

This test is checking for the validity of the path used as parameter
for mkdtempSync.

PR-URL: https://github.com/nodejs/node/pull/12009


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>