test: allow testing uid and gid separately

This commit lets the uid and gid options of spawn() to be
tested independently of one another based on the user's uid
and gid.

Fixes: https://github.com/nodejs/node/issues/10646
PR-URL: https://github.com/nodejs/node/pull/10647


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>