How can I login as admin after redirect to custom login page

There is no reason to redirect the logged-in users. So I would recommend that you add another conditional and redirect the users only if they are not logged in:

add_action('init','possibly_redirect');
function possibly_redirect(){
    global $pagenow;
    if( 'wp-login.php' == $pagenow && ! is_user_logged_in()) {
        wp_redirect('http://localhost/test/login/');
        exit();
    }
}

This way you won’t be redirected after login.