test: increase coverage for stream's duplex

Make use of Arrow function.
Add a small test and this file's coverage is 100%.
https://github.com/nodejs/node/blob/a647d82f83ad5ddad5db7be2cc24c3d686121792/lib/_stream_duplex.js#L25
Coverage: https://coverage.nodejs.org/coverage-067be658f966dafe/root/_stream_duplex.js.html

PR-URL: #10963
Reviewed-By: Colin Ihrig <[email protected]>
Reviewed-By: Anna Henningsen <[email protected]>
Reviewed-By: Italo A. Casas <[email protected]>
Reviewed-By: James M Snell <[email protected]>
Reviewed-By: Michael Dawson <[email protected]>
Reviewed-By: Luigi Pinca <[email protected]>
Reviewed-By: Matteo Collina <[email protected]>

Author: hiroppy

test/parallel/test-stream-duplex.js

@@ -5,27 +5,28 @@ const Duplex = require('stream').Transform;
 
 const stream = new Duplex({ objectMode: true });
 
+assert(Duplex() instanceof Duplex);
 assert(stream._readableState.objectMode);
 assert(stream._writableState.objectMode);
 
 let written;
 let read;
 
-stream._write = function(obj, _, cb) {
+stream._write = (obj, _, cb) => {
   written = obj;
   cb();
 };
 
-stream._read = function() {};
+stream._read = () => {};
 
-stream.on('data', function(obj) {
+stream.on('data', (obj) => {
   read = obj;
 });
 
 stream.push({ val: 1 });
 stream.end({ val: 2 });
 
-process.on('exit', function() {
+process.on('exit', () => {
   assert.strictEqual(read.val, 1);
   assert.strictEqual(written.val, 2);
 });