test: deflake cluster-concurrent-disconnect

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

Fixes: nodejs#38405

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(() => {