Huggietoes, I am not an authority on these things, and reading your post, I feel your pain, so my suggestion would be to go to the Nursing Manager and ask again.
Is it possible to have someone else switch shifts with you for those few days??
If not, then they are making it impossible for you to get what you need. If you are tired, stressed, etc when you take the test then you may not do as well, and you have worked too hard and owe it to yourself to do what is best for you!
Do you really want to work for a employer that already shows you that they care nothing for your needs?
Point out all that you have said here and stay firm......
I wish you the best of luck with this and the exam!