test: refactor test-child-process-env to use arrow functions

In `test/parallel/test-child-process-env.js`, callbacks use
anonymous closure functions. It is safe to replace them with arrow
functions since these callbacks don't contain references to `this`,
`super` or `arguments`. This results in shorter functions.

test/parallel/test-child-process-env.js

@@ -48,12 +48,12 @@ let response = '';
 
 child.stdout.setEncoding('utf8');
 
-child.stdout.on('data', function(chunk) {
+child.stdout.on('data', (chunk) => {
   console.log(`stdout: ${chunk}`);
   response += chunk;
 });
 
-process.on('exit', function() {
+process.on('exit', () => {
   assert.ok(response.includes('HELLO=WORLD'));
   assert.ok(response.includes('FOO=BAR'));
   assert.ok(!response.includes('UNDEFINED=undefined'));