Prevent "testing" user from being on packet

Add some check when populating `signatures_upper` to prevent adding the user `testing` so that we don't have to keep deleting it.

Inspect the code that populates `signatures_upper` and determine how the `testing` user is added to a packet. Verify the resulting packet no longer includes that user, and add or update coverage if the relevant test location is available; done means `testing` is consistently excluded without manual deletion.

Rédigé par le modèle d'indexation à partir du texte de l'issue.