I was also scared of my TOEFL exam. Before I started, I told the examiner that I have a stutter and that I might need a little extra time. He was very friendly and told me not to worry. So we talked about the chosen topic, he asked a few questions and it felt more like a discussion than an actual exam. It was over in no time. It helped a lot letting him know beforehand because it took the pressure off (I was „allowed“ to stutter and I didn‘t have to try to hide it or fear an unpleasant reaction). Good luck!