test: replace error msg w/ template literal

PR-URL: https://github.com/nodejs/node/pull/15910
Reviewed-By: Daijiro Wachi <daijiro.wachi@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>

test/parallel/test-cluster-send-handle-twice.js

@@ -35,8 +35,8 @@ if (cluster.isMaster) {
   for (let i = 0; i < workers.toStart; ++i) {
     const worker = cluster.fork();
     worker.on('exit', common.mustCall(function(code, signal) {
-      assert.strictEqual(code, 0, 'Worker exited with an error code');
-      assert.strictEqual(signal, null, 'Worker exited by a signal');
+      assert.strictEqual(code, 0, `Worker exited with an error code: ${code}`);
+      assert.strictEqual(signal, null, `Worker exited by a signal: ${signal}`);
     }));
   }
 } else {