Just replying to reiterate what everyone else has already said, really - and to let you know you are definitely not alone with the obsessive/irrational thoughts. They are so tough to deal with and I have been going through the exact same thing recently. Nobody seems to get it, and it's tough because no matter how much you KNOW, or how much you tell yourself "I'm fine", that tiny voice of doubt at the back of your mind saying "yes, but this could really be something badů" just doesn't go away.
I have started working through the Health Anxiety workbook (there's a sticky topic at the top of this page) that is pretty useful in helping getting a handle on the actual anxiety.
I know you will feel just fine after the test and when you see for sure you don't have it, just try and hang in there in the meantime, I wish I had an easy answer as to how to just 'stop worrying' (because then I could stop myself!) but sadly I don't, know you are definitely not alone though. :)