So first off, drug testing is when they have you pee in a cup and test if you've been taking drugs. There have been court issues where people are saying it violates they're 4th and 5th amendment rights.
The 8th amendment protects people from "Cruel and Unusual Punishment" and "excessive fines, bail, and punishment". But "Excessive" or "Cruel and Unusual" is not clearly defined. What is your definition?