test: use arrow functions in callbacks

PR-URL: #24441
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>

test/parallel/test-stream-pipe-await-drain.js

@@ -22,22 +22,22 @@ writer1._write = common.mustCall(function(chunk, encoding, cb) {
  cb();
 }, 1);
 
-writer1.once('chunk-received', function() {
+writer1.once('chunk-received', () => {
  assert.strictEqual(
  reader._readableState.awaitDrain,
  0,
  'awaitDrain initial value should be 0, actual is ' +
  reader._readableState.awaitDrain
  );
- setImmediate(function() {
+ setImmediate(() => {
  // This one should *not* get through to writer1 because writer2 is not
  // "done" processing.
  reader.push(buffer);
  });
 });
 
 // A "slow" consumer:
-writer2._write = common.mustCall(function(chunk, encoding, cb) {
+writer2._write = common.mustCall((chunk, encoding, cb) => {
  assert.strictEqual(
  reader._readableState.awaitDrain,
  1,
@@ -49,7 +49,7 @@ writer2._write = common.mustCall(function(chunk, encoding, cb) {
  // will return false.
 }, 1);
 
-writer3._write = common.mustCall(function(chunk, encoding, cb) {
+writer3._write = common.mustCall((chunk, encoding, cb) => {
  assert.strictEqual(
  reader._readableState.awaitDrain,
  2,