Since you are using AD as your authentication method, it is likely that you will have to add a dummy user to that system in order for the user to be able to log on to the Server. Once you have added the user, you will just need to ensure that this user doesn't have admin rights on the Server so that you can easily vet whether or not the permissions are setup correctly.
Hope this helps,
Yeah, I know about that. Was hoping for a backdoor user
Were you ever able to setup a dummy user bypassing the AD Group method? I face same challenges of validating permissions, in such scenario a test user comes in handy.
I use our Run As User account. It's set up with a non-admin site role (Interactor, Publisher, or Viewer) as necessary. Trying to get an AD account to use as a dummy/test account wasn't allowed at the time (5 year ago!) and the Run As User acct. works fine for my puposes.