You only need to add the page slug to “show_in_menu”, making the answer to your example:
'show_in_menu' => 'mytestpage',
For a more complete example:
Admin/Settings page of: /wp-admin/admin.php?page=mytestpage
register_post_type('mycpt', array('label' => 'My Custom Post Type', 'capability_type' => 'post', 'show_ui' => true, 'show_in_menu' => 'mytestpage'));