The emergency stop is a standard part of every test. The examiner will tell you when you get to that part so you will have a bit of pre-warning and will be in complete control of the situation - and you won't be going very fast either. My friend passed her test when she was pregnant and was fine.
I was in the car with her a couple of weeks ago when she had to do an emergency stop and the seatbelt didn't actually tighten over my belly, there was just a bit of pressure on the bit over my chest as I leaned forward.
You would only need to worry if you actually hit something, but even then it would be more your chest than your belly that would get the most of the impact. About 8yrs ago I crashed into the back of another car while doing about 45/50mph and I had a massive black bruise on my boob where the seatbelt dug in, but no injury to my stomach area.
I hope everything goes well with your test - fingers crossed for you xxxxx