test: updated error message

PR-URL: https://github.com/nodejs/node/pull/15906
Reviewed-By: Daijiro Wachi <daijiro.wachi@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>

test/parallel/test-cluster-send-deadlock.js

@@ -31,8 +31,8 @@ const net = require('net');
 if (cluster.isMaster) {
   const worker = cluster.fork();
   worker.on('exit', function(code, signal) {
-    assert.strictEqual(code, 0, 'Worker exited with an error code');
-    assert(!signal, 'Worker exited by a signal');
+    assert.strictEqual(code, 0, `Worker exited with an error code: ${code}`);
+    assert(!signal, `Worker exited by a signal: ${signal}`);
     server.close();
   });