I would recommend mentioning that you stutter very briefly in the beginning of the first question you answer. Something along the lines of "I have a stammer which may impact my answering ability for this part of the test" On the other hand, when you apply to colleges you can let them know that your Speaking section suffered because of your stutter and they should be accommodating. I don't know if my example would help or not, but I was in the same situation a couple of years, bombed the speaking section got 18/30 but still got accepted to a top 20 school in my field.