What do I need for testing a single script in admin

Sounds to me like you’re trying to create a custom Admin Page. Instead of adding a file into the admin folder, you can create custom admin pages from your WordPress plugin or theme by using the ‘Admin Menu’ action.

In short, you will need to add something like the following to your plugin, or your theme’s functions.php file.

// Create the page
function my_plugin_menu() {
    add_options_page( 'My Plugin Options', 'My Plugin', 'manage_options', 'my-unique-identifier', 'my_plugin_options' );
}
add_action( 'admin_menu', 'my_plugin_menu' );

// Define the page content 
function my_plugin_options() {
    if ( !current_user_can( 'manage_options' ) )  {
        wp_die( __( 'You do not have sufficient permissions to access this page.' ) );
    }
    echo '<div class="wrap">';
    echo '<p>Here is where the form would go if I actually had options.</p>';
    echo '</div>';
}

For full details on how to do this, please view the section about Admin Pages on the WordPress Codex.
https://codex.wordpress.org/Adding_Administration_Menus

I hope that helps!