test: use error code rather than message in test

Use err.code checking instead of err.message checking in
test-child-process-fork-closed-channel-segfault.

PR-URL: #20859
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>

Author: Trott

test/parallel/test-child-process-fork-closed-channel-segfault.js

@@ -66,7 +66,7 @@ const server = net
         send(function(err) {
           // Ignore errors when sending the second handle because the worker
           // may already have exited.
-          if (err && err.message !== 'Channel closed') {
+          if (err && err.code !== 'ERR_IPC_CHANNEL_CLOSED') {
             throw err;
           }
         });