I assume it's not possible unless you know the in-memory state of all the other gateway routers on the internet, no? You can know what they advertise, but that's not the same thing as a full description of their internal state and how they will choose to update if a route gets withdrawn.

I think you could know the state of the peers and simulate what they advertise and receive and validate that. The test unit would need to be a simulated router that behaves exactly as the real one, I actually think its technically doable with tight version control for routers.