Dude can't test you for drugs without consent. Your employer has to inform you of the drug test and have you sign a waiver (whichis sometimes agreed to in the employment contract for the length of your stay) before you can legally be tested.

But if you're really that concerned about it, just stop puffin' for a couple of weeks. You say you will have 3 weeks approximately, that is most likely enough time to detox so you won't have to worry about bringing some fake piss in you ass cheeks. Or you could drink the sludge... then your worries are mute again.

Good luck.