I had the same fears going in, but I informed the examiner of my stutter just before the speaking test and he was so cool about it. It will be you and the examiner alone in a room with no external pressure. Just remember to not say filler words like “uhhh” or “umm” and you’ll be set. Remember that these examiners spoke to hundreds/thousands of people, you will certainly not be the first person with a stutter they see.