What company is doing anything interesting that drug tests? If you tell anyone at the company that you have in the past or do now do drugs or try to stall you won't get the job. Second, who cares if they reneg on the job? There are literally tens of thousands of companies at which to work. Every single company on the face of the planet wants CS people. The only course of action if you want the job is don't say anything and just do the test. Otherwise don't waste your time or theirs.