Hi Alyssa!
Yikes, sounds like you have quite the situation.
Take the test, ok? You don't have to worry about how to tell them until you know for sure.
Actually, you might still want to consider writing them a letter and then leaving for a few hours to give them time to get out their first emotions.
Sometimes what helps is to say things like how scared you are and stuff like that.
If you don't want to write a letter you could ask to go out to supper with them and then tell them at the restaurant (maybe write them a note and have them read it there) because they can't act TOO bad in a restaurant.

No matter how you decide to tell them, I promise that you will feel ten times better when they know and you don't have to hide it.
But like I said, no need to worry about it until you take the test.

Good luck, hun! Let us know how it goes!
P.S. That must have been so hard! You said that your parents still haven't gotten over it, but how are you doing?