The Drupal tests are passing but composer tests don't pass because it loads t...

The Drupal tests are passing but composer tests don't pass because it loads the scaffold plugin from here: https://github.com/drupal/core-composer-scaffold. How do we get past this failure?

The only idea I have is to define a path repository in those composer.json files, but I am not sure if that is an acceptable approach. It may not be acceptable if those composer.json is meant to be a template for users and not only for the testing script.