add custom order status

 
function register_custom_order_status() {
    register_post_status( 'x-test', array(
        'label'                     => 'Test',
        'public'                    => true,
        'exclude_from_search'       => false,
        'show_in_admin_all_list'    => true,
        'show_in_admin_status_list' => true,
        'label_count'               => _n_noop( 'Test <span class="count">(%s)</span>', 'Test <span class="count">(%s)</span>' )
    ) );
}
add_action( 'init', 'register_custom_order_status' );

// Add to list of WC Order statuses
function add_custom_order_statuses( $order_statuses ) {

    $new_order_statuses = array();

    // add new order status after processing
    foreach ( $order_statuses as $key => $status ) {

        $new_order_statuses[ $key ] = $status;

        if ( 'wc-processing' === $key ) {
            $new_order_statuses['x-test'] = 'Test';
        }
    }

    return $new_order_statuses;
}
add_filter( 'wc_order_statuses', 'add_custom_order_statuses' );

 case 'x-test':
        case 'wc-x-test':
            return 'success';
            break;