test: replace callback with arrow function

PR-URL: nodejs#24531
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

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

@@ -29,7 +29,7 @@ cat.stdout.on('end', common.mustCall());
 cat.stderr.on('data', common.mustNotCall());
 cat.stderr.on('end', common.mustCall());
 
-cat.on('exit', common.mustCall(function(code, signal) {
+cat.on('exit', common.mustCall((code, signal) => {
   assert.strictEqual(code, null);
   assert.strictEqual(signal, 'SIGTERM');
 }));