test: add test case of PassThrough

This commit adds the test case of PassThrough.
This test case checks that PassThrough can
construct without new operator.

This is a part of Code And Learn at NodeFest 2016

Fixes: nodejs/code-and-learn#58
PR-URL: #9581
Reviewed-By: Yosuke Furukawa <yosuke.furukawa@gmail.com>
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>

Author: kt3k

test/parallel/test-stream2-transform.js

@@ -106,6 +106,14 @@ test('object passthrough', function(t) {
   t.end();
 });
 
+test('passthrough constructor', function(t) {
+  const pt = PassThrough();
+
+  assert(pt instanceof PassThrough);
+
+  t.end();
+});
+
 test('simple transform', function(t) {
   var pt = new Transform();
   pt._transform = function(c, e, cb) {