You are not required to carry or use one in a test.
Every horse is different, I currently carry one but I do not use it. My horse is pretty terrified of whips, I have de-sensitized him on the ground and now I am working on it under-saddle. If I was to actually use it, I'd be a bloody splotch 2 counties over. He does not need me to use it, he was lazy and lacking in impulsion so my trainer and I worked on that until he no longer was. I am working on the de-sensitization because he was having issues with other riders that were carrying one, passing him.
Personally, I prefer a horse that does not *need* one. It's just one more thing to remember, haul around and have to deal with.