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: nodejs#9578
Reviewed-By: Yosuke Furukawa <[email protected]>
Reviewed-By: Shigeki Ohtsu <[email protected]>
Reviewed-By: Colin Ihrig <[email protected]>
Reviewed-By: Anna Henningsen <[email protected]>

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

@@ -11,7 +11,7 @@ var nChunks = 10;
 var chunk = Buffer.alloc(10, 'x');
 
 r._read = function(n) {
-  setTimeout(function() {
+  setImmediate(function() {
     r.push(--nChunks === 0 ? null : chunk);
   });
 };