You live in their house. Move out or just take the test. If you only smoke weed, then you will test positive for thc and nothing else. Even if your parents are the most misinformed, ignorant, idiots, they will at least understand that you're smoking pot and not dropping E or coke or anything. Also I dont care how narrow minded you are, everyone knows that marijuana is a lower class of drug and is less harmful than extacy for example.
Make weed brownies and just leave them lying around. Don't say "I just made brownies," just leave them and go watch TV or do something else. Or better yet, leave for like, an hour, then come back and see if any are missing. The next day you can be like "That wasn't so bad, was it?"
Which was when, last week? Shut the hell up. Sounds like attempting to reason with your parents is pointless. Maybe just try picking her brain about it next time in a civil conversation. Ask her why she feels the way she feels. Take her answers, and give some thought to your counter. Don't stand up immediately and start defending it. Then, in a few days time, present your counter in a calm way.