test: use duplex streams in duplex stream test

test-stream-duplex.js uses transform streams instead of the
duplex stream base class. This leads to some code not being
covered in the Duplex constructor.

PR-URL: https://github.com/nodejs/node/pull/12514
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Yuta Hiroto <hello@about-hiroppy.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: David Cai <davidcai1993@yahoo.com>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>

test/parallel/test-stream-duplex.js

@@ -1,7 +1,7 @@
 'use strict';
 require('../common');
 const assert = require('assert');
-const Duplex = require('stream').Transform;
+const Duplex = require('stream').Duplex;
 
 const stream = new Duplex({ objectMode: true });