test: improve assert message

PR-URL: #15909
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>

test/parallel/test-cluster-worker-destroy.js

@@ -49,7 +49,7 @@ if (cluster.isMaster) {
     });
 
     const w = cluster.worker.disconnect();
-    assert.strictEqual(w, cluster.worker, 'did not return a reference');
+    assert.strictEqual(w, cluster.worker);
   } else {
     // Call destroy when worker is not disconnected yet
     cluster.worker.destroy();