A bit of overanalysis paralysis in this thread IMO. Yeah caffeine has been shown to provide slight boosts in athletic performance but it’s not going to make the difference between passing and failing. Maybe it helps you crank out 5 extra pushups and shave 10 seconds off your mile. To me this implies two things:
(1) It’s neither “unfair” nor does it defeat the purpose of the fitness test. If you’re out of shape, you won’t pass just because you chugged some caffeine beforehand. And it’s not giving you any unfair advantage because it’s not prohibited and caffeine is available for any other candidates too (It’s not like this is some secret super-soldier serum from Captain America).
(2) If it makes any difference it’s gonna be minor. Like 10 points on the SAT, if that. In other words it probably doesn’t matter much either way, but if you want to leave no stone unturned and feel like it will help, go for it.
By the way, please check with your doctor before taking energy drinks. And read the back of the label of anything you consume. Plenty of energy drinks are fine but some of those brands pack some nasty stuff. Good luck with your CFA!