test: deflake cluster-concurrent-disconnect

Occasionally the error code is `'ENOTCONN'` on macOS. Add it as an
allowed/expected code.

Fixes: #38405

Author: lpinca

test/parallel/test-cluster-concurrent-disconnect.js

@@ -26,7 +26,7 @@ if (cluster.isPrimary) {
     // to send messages when the worker is disconnecting.
     worker.on('error', (err) => {
       assert.strictEqual(err.syscall, 'write');
-      assert.strictEqual(err.code, 'EPIPE');
+      assert(['EPIPE', 'ENOTCONN'].includes(err.code), err);
     });
 
     worker.once('disconnect', common.mustCall(() => {