test: use .code for error in setgid

When the 'nobody' user is missing use .code to detect this, its more
robust than than the .message string.

Refs: #19594

PR-URL: #28219
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Yongsheng Zhang <zyszys98@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Yuta Hiroto <hello@hiroppy.me>
Reviewed-By: Anto Aravinth <anto.aravinth.cse@gmail.com>

test/parallel/test-process-uid-gid.js

@@ -74,7 +74,7 @@ const oldgid = process.getgid();
 try {
   process.setgid('nobody');
 } catch (err) {
-  if (err.message !== 'setgid group id does not exist') {
+  if (err.code !== 'ERR_UNKNOWN_CREDENTIAL') {
     throw err;
   }
   process.setgid('nogroup');