LiamKenyon the way the Flarum permission works, you can't "be" a permission, you must be a user, and it must exist in the database so each extension can query relationships off that users for all the access logic that isn't based on permissions or is combined with permissions.
You would have to create a real (maybe temporary) user to achieve this. In fact you can already do exactly that using the native Flarum, just create a new user you use for testing. To make accessing your test users easier without requiring a lot of logging out and password copy you could use this existing extension https://discuss.flarum.org/d/9868-fof-impersonate-login-as-other-users
It's theoretically possible to create a virtual user and send it though the permission system, but this will be a nightmare of extension compatibility. It's almost certain the logic will break sooner or later when trying to use it together with any sophisticated community extension. Maintaining a virtual user extension that works with all community extensions would probably be a massive never-ending work, so I only see this as a potential paid extension.
It would be much easier for an extension to automate the creation of real test users. But do you really need to create test users on the fly? For most use cases it seems more logical to me to setup a real test user account and re-use that account for future tests. That's what I do on my development forums, I usually have one admin, one moderator and one regular users and I always use all 3 in different browser contexts to try out features.
