#67 should have fixed this but some tests to cover our back for regressions wouldn't harm.

I think the general idea would be to add a custom CPT like this:

$args = array(
   'public' => false,
   'show_ui' => true,
   'label'  => 'Test No Metabox'
);
register_post_type( 'test_no_metabox', $args );

from here

And then run a simple test on that CPT.