On the other hand, just as with database backups, making them is only half of the story. You have to test restores/recovery. Does your plan actually work? What have you overlooked? What edge cases do you need to accommodate?

Many data centers will test backup power generation regularly just for this reason. It's not unheard of at all and the risk of a problem at a planned time is worth the confidence in knowing that the system is more likely to work when needed at an unexpected time.