I've done this before. Not sure if it's still the same but back when I did it, the speaking part of the exam was a one-to-one conversation which is recorded by the examiner. What I did was tell the examiner about my stutter before he started recording, while he was explaining how it works. As far as I remember, he just made a note to indicate the problem and told me not to worry about it and just speak as I would on a normal conversation. Obviously Speaking wasn't my best score but didn't keep me from passing the exam with a decent grade on the first try. I'm sure you can do the same!