Don't be nervous. Just study a little before the test so you're fresh on your knowledge and do your best. I tested into second year Japanese when I entered college (after only self-teaching myself before that), but a lot of people don't. Even if you have to start with first-year Japanese again, don't look at it as a bad thing: it'll strengthen your knowledge of the basics (and you doubtlessly have a few wholes in your basic knowledge due to self-teaching; everyone who does it does), and you'll probably end up one of the strongest Japanese students in your class thanks to it.
Whatever happens, it will likely be for the best, so don't stress!