Police officers working undercover have exceptions from certain criminal laws. For instance, law enforcement officers directly engaged in the enforcement of controlled substance laws are exempt from laws surrounding the purchase, possession, sales or use of illegal substances. ref. 21 U.S.C. § 885(d) of the U.S. Code.

the only person who can order a cop to take a drug test is IA. for them to order it, there would be a case against the cop and he wouldn't be working undercover.