I wouldn’t treat it as a practice test, most, if not all, of the other people taking the test will be taking it for real. If you’ve done well in the course and know the material, I’m not sure the reason for your concern. If you’re that uncomfortable taking it, I would not take it, and take another subject test or none at all. One subject test won’t make that much difference, esp since you have the 800 in math 2.