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.
Fixes: https://github.com/nodejs/node/issues/13071
Checklist
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make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
commit message follows commit guidelines
Affected core subsystem(s)
test process