test: use setImmediate() in test of stream2

use setImmediate() insted of setTimeout()
 in test of stream2 push.
The test is in test/parallel/test-stream2-push.js

Fixes: nodejs/code-and-learn#58
PR-URL: nodejs#9583
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Shigeki Ohtsu <ohtsu@ohtsu.org>

test/parallel/test-stream2-push.js

@@ -111,7 +111,7 @@ function end() {
   source.emit('end');
   assert(!reading);
   writer.end(stream.read());
-  setTimeout(function() {
+  setImmediate(function() {
     assert(ended);
   });
 }