If the test comes back positive does the fire dept. allow you to go to rehab and still keep your job? It seems to me that they wouldnt fire someone just for one bad test.
Also, be careful what you wish for. Why not just quit now before you reach a bad bottom, it can be done and there are many people here who have done it. I would say just the chance of losing your dream job is enough of a bottom to seriously start looking for help. Reaching out here is a great first step.