This is a very simple example that can be used to test CORS.
It will accept GET, POST & OPTIONS Requests.
It's written in PHP, just because it is simple to use
If you are not familiar with PHP, no worries, it's easy to run.
In the directory the file is located just run
sudo php -S localhost:3000
PHP will default to the index.php file, so if you just browse to localhost, it will find this file.
Happy CORS