test: replace anonymous closure with arrow func

PR-URL: #24480
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

test/parallel/test-stream2-unpipe-drain.js

@@ -51,13 +51,13 @@ const src2 = new TestReader();
 
 src1.pipe(dest);
 
-src1.once('readable', function() {
-  process.nextTick(function() {
+src1.once('readable', () => {
+  process.nextTick(() => {
 
     src2.pipe(dest);
 
-    src2.once('readable', function() {
-      process.nextTick(function() {
+    src2.once('readable', () => {
+      process.nextTick(() => {
 
         src1.unpipe(dest);
       });
@@ -66,7 +66,7 @@ src1.once('readable', function() {
 });
 
 
-process.on('exit', function() {
+process.on('exit', () => {
   assert.strictEqual(src1.reads, 2);
   assert.strictEqual(src2.reads, 2);
 });