You don't have to stop taking phentermine. You're not doing anything wrong. Don't worry
It can (probably will) show a false positive for amphetamines. It doesn't matter as long as you have a prescription for phen.
Before the test someone usually talks to you about if you're taking any prescription drugs. Just tell the person you are taking the diet drug, phentermine. They might ask you to verify this. I'd just bring the bottle with me so you can show it there and not have to send something in later. It's no big deal and the bottle is proof of your prescription because it has all the information on it.