We have a lot of customers who maintain a separate test instance of ProGet; while upgrade testing is important of course, a dedicated testing instance also lets you evaluate new ProGet feature usage patterns (such as requiring promotion workflows, etc.), try out new tools (perhaps new version of visual studio, etc.), and conduct training on ProGet usage -- all without risking/disturbing your production instance.
To keep things simple from a licensing perspective, we just treat testing instances separate instances (and thus require a separate license key). Many customers use a ProGet Free License for this, but of course not all the features are available. It's rare to see a second license be cost prohibitive, especially given the labor/server costs involved with maintaining a testing instance -- even ProGet Enterprise customers will have full instances just for testing and even DR purposes.
You're right --- Active Directory is usually a pain point; sometimes our code changes (we try to never touch this), but also people want to change their AD configuration (move to LDAPS, etc.). Wrong settings, and you can lock-out your instance. If it's an uncommon / one-off testing case then a temporary trial license is fine for this.