test: fix test-cluster-worker-init.js flakyness

Update test to match current test guidelines and use common.mustCall
instead of unref'd timer.

PR-URL: https://github.com/nodejs/node/pull/8703
Fixes: https://github.com/nodejs/node/issues/8700
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>

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

@@ -3,30 +3,25 @@
 // verifies that, when a child process is forked, the cluster.worker
 // object can receive messages as expected
 
-require('../common');
-var assert = require('assert');
-var cluster = require('cluster');
-var msg = 'foo';
+const common = require('../common');
+const assert = require('assert');
+const cluster = require('cluster');
+const msg = 'foo';
 
 if (cluster.isMaster) {
-  var worker = cluster.fork();
-  var timer = setTimeout(function() {
-    assert(false, 'message not received');
-  }, 5000);
+  const worker = cluster.fork();
 
-  timer.unref();
-
-  worker.on('message', function(message) {
-    assert(message, 'did not receive expected message');
+  worker.on('message', common.mustCall((message) => {
+    assert.strictEqual(message, true, 'did not receive expected message');
     worker.disconnect();
-  });
+  }));
 
-  worker.on('online', function() {
+  worker.on('online', () => {
     worker.send(msg);
   });
 } else {
   // GH #7998
-  cluster.worker.on('message', function(message) {
+  cluster.worker.on('message', (message) => {
     process.send(message === msg);
   });
 }