parallel/test-process-setuid-setgid fails if "nobody" user does not exist

[Qantas94Heavy]

• Version: 7.10.0
• Platform: Linux
• Subsystem: test

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

[gibfahn]

Test was re-enabled in #12403, so cc/ @Trott

[Trott]

@Qantas94Heavy Are you running tests as superuser or as an unprivileged user? Can you copy/paste the test failure output?

[Trott]

Fix (for the non-super user situation only) in #13112. @Qantas94Heavy If you could test it, that would be great. Thanks.