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 <cjihrig@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Italo A. Casas <me@italoacasas.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>

Author: hiroppy

test/parallel/test-stream-duplex.js

@@ -5,27 +5,28 @@ const Duplex = require('stream').Transform;
 
 conststream=newDuplex({objectMode: true});
 
+assert(Duplex()instanceofDuplex);
 assert(stream._readableState.objectMode);
 assert(stream._writableState.objectMode);
 
 letwritten;
 letread;
 
-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);
 });