Woocommerce API call back is not accepting POST parameters
add_action('init', 'test_url_callback');add_action('woocommerce_api_test_url', array( $this, 'test_url_callback' ) );Please add add_action('init', 'test_url_callback'); before your action then you can get POST parameters with $_POST in your test_url_callback function.
If parameters sent as json, you can get them with;
$data = json_decode(file_get_contents('php://input'), true);