OK, I reread your original post and it sounds like this is a CURRENT job. That changes my answer...
This SHOULD be how this goes down: if you test positive for THC, which you likely will, the testing facility should call you first to ask you if you know of any reason why you would have a positive test. I would be completly honest with them, start with the benzo script info and then let them know you are and I quote "struggling to stop your use of marijuana for recreational purposes." As soon as you hang up the phone look up a phone number for an outpatient rehab facility and make an appointment. When your employer calls you into the office and asks about the drug fail you tell him you struggle with addiction issues and you are CURRENTLY seeking treatment. Apologize for the issues and try to get them to ACKNOWLEDGE that your work performance has not been affected by the PAST substance use. My capital words are on purpose because that is the way it is spelled out by law. If you are CURRENTLY seeking help for drug use prior to termination ( I know I know it's just weed I didn't write the laws ) and it's not affecting your work performance in a negative way then your employer is opening themselves up a lawsuit.
Not a fun experience but the above is better than losing one's home ( I suggest you mention that part too )
In addition, if the employer has not given you a handbook or detailed the company drug policy you can claim ignorance if you do lose your job and likely gain the opportunity for unemployment. It's not an easy argument but it can happen, I've seen it.
Research employee fairness laws with regard to drug testing and make your case. It would not be a bad idea to make an appointment now for outpatient treatment as they will fuck themselves if they fire an employee with chemical dependence, a legal disability, while they are seeking treatment.
That'll be $500 a quarter of your finest...