Some degree of stress prior to important events is actually healthy - it kick starts us into action, whether it be to study more or whatever.

I'd highly recommend you find some breathing time prior to writing tests. You need 3 minutes of slow steady breathing in and out. It does help. And whilst you're writing stop and pause, catch your breath and focus on your breathing for a minute breathing slowly in and out. It's something no one will notice and you'd rather take the one minute time out than miss the plot with everything.

The other thing is affirmations whilst you're writing.

If you become stuck on a question and truly think hell I don't know ... Tell yourself "I can't figure this out right now, but my brain does know this answer. I will come back to this question and work it out". And then move onto the next question you do know (don't forget to go back to unanswered questions)