test: allow for absent nobody user in setuid test

Some isolated build or test VMs don't have a "nobody" user, causing the
parallel/test-process-setuid-setgid test to fail. Add logic to allow for
that situation.

PR-URL: https://github.com/nodejs/node/pull/13112
Fixes: https://github.com/nodejs/node/issues/13071


Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>