Where is the php file, that does the checks for login information?

You don’t want to be messing around with that.
Look at the following hook:
https://codex.wordpress.org/Plugin_API/Action_Reference/wp_login

You should do something like:

function check_test_user() {
// Your code here
}

add_action('wp_login', 'check_test_user');

The hook will run as part of the Login process and saves you modifying core WordPress files.