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>

test/parallel/test-process-setuid-setgid.js

@@ -51,7 +51,7 @@ if (process.getuid() !== 0) {
 
   assert.throws(
     () => { process.setuid('nobody'); },
-    /^Error: EPERM, /
+    /^Error: (EPERM, .+|setuid user id does not exist)$/
   );
   return;
 }