The add_meta_boxes
hooks load in edit-form-advanced.php
. Your custom plugin page “test” does not use that file. You need to hook into the “plugin admin page hooks” instead.
Secondly, the third parameter of add_meta_box
is just the CPT slug, not the page slug you have.
Here is what you need to change:
add_action("load-my-cpt_page_my-cpt-slug", "my_add_metaboxes");
function my_add_metaboxes()
{
add_meta_box("my-cpt-metabox", "My CPT Metabox", "display_my_metabox", "my-cpt", "normal", "core");
}