test: change from setTimeout to setImmediate

This is a part of Code And Learn at NodeFest 2016 Challenge

Fixes: nodejs/code-and-learn#58
PR-URL: #9578
Reviewed-By: Yosuke Furukawa <yosuke.furukawa@gmail.com>
Reviewed-By: Shigeki Ohtsu <ohtsu@ohtsu.org>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>

test/parallel/test-stream-unshift-empty-chunk.js

@@ -12,7 +12,7 @@ var chunk = new Buffer(10);
 chunk.fill('x');
 
 r._read = function(n) {
-  setTimeout(function() {
+  setImmediate(function() {
     r.push(--nChunks === 0 ? null : chunk);
   });
 };