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: #12514
Reviewed-By: James M Snell <[email protected]>
Reviewed-By: Yuta Hiroto <[email protected]>
Reviewed-By: Luigi Pinca <[email protected]>
Reviewed-By: David Cai <[email protected]>
Reviewed-By: Daniel Bevenius <[email protected]>

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 });