At the doctors offices I worked at, it was water and black coffee (no sugar, no cream, etc) only; tea was not allowed. But, it’s… a bit of an inexact science, and I think the coffee was more that you can’t really take away coffee in the morning from Americans if you don’t want to have bigger problems (like violent fights breaking out in the waiting room, or everyone being hours late for their testing every day); the way it was framed to me always made me think they’d be saying “water and black tea only (no sugar, no cream)” if we lived in a country where tea was the dominant drink and not coffee. Either way, unless this is a big test and a few points could make a big difference for you, it’s probably going to be ok.