test: fix flaky cluster unix socket test

Ensure `common.tmpDir` exists before trying to chdir into it.  Fixes a
"ENOENT: no such file or directory, uv_chdir" error when the temporary
directory is removed before running the test.

PR-URL: https://github.com/nodejs/node/pull/18263


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Jon Moss <me@jonathanmoss.me>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Minwoo Jung <minwoo@nodesource.com>