test: improve template value for test message

Include value that cause failure in error message in
test-cluster-master-kill.js.

PR-URL: https://github.com/nodejs/node/pull/16826
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>

test/parallel/test-cluster-master-kill.js

@@ -81,7 +81,8 @@ if (cluster.isWorker) {
   process.once('exit', () => {
     assert.strictEqual(typeof pid, 'number',
                        `got ${pid} instead of a worker pid`);
-    assert.strictEqual(alive, false, 'worker was alive after master died');
+    assert.strictEqual(alive, false,
+                       `worker was alive after master died (alive = ${alive})`);
   });
 
 }