Rusfox
Don't you have any friends who are older then you? You can ask anybody older you to buy the test for you.
Think of any adult you can trust and ask her\him to buy it for you, there is nothing to be ashaimed of here, we all have been teenagers and fully understand your worries.
After all you can ask your boyfriend to buy it for you.
You need to know what to expect.
And don't worry in any case. Your parents are your parents - not your enemies. Who else but them want the best for you.
They probably were trying to frighten you in different ways just to make sure you will be careful.
Everything is going to be fine!
Good luck!