I took the IELTS 2 years ago to get into a Dutch uni. I will preface this by saying that my stutter is not that bad most of the time, I will only get stuck once in a while. I did get stuck once during the speaking part, but the lady did not even flinch and I got an 8.5. You can also tell the person that you have a stutter beforehand, so that they know. But in my experience, it really shouldn’t matter.