Doesn't that sort of kill the point of legalising drugs?
I'd like to see drugs legalised, because I think people should be allowed to consume anything they like. If they choose not to be properly informed about the drug and they take too much of it, then great. One less idiot. If you can't handle the drugs, then you shouldn't be taking them. I can't handle drugs. I don't take them. People don't need to be babysat. The information is freely available about drugs. If you're unsure about whether or not you will be able to take drugs, you can voluntarily take a variety of tests similar to what you have suggested.
Your plan of regulation would mean that those who don't qualify from your series of tests will just continue to buy it illegally. It's the junkies who would not be deemed fit by the test who are the criminals' best customers. You do all those tests and then the purchaser just shares his drugs with his friends who don't need to go through the test.
If I wanted to buy any drug, I think I'd have a good chance of getting it in the space of 3 hours. Drugs are freely available, but illegal. The money goes to the criminals and it's all done cloak and dagger, when they could be legal and the money would go to private entrepreneurs and the government in tax.
I'm not seeing any justifiable reason why cigarettes, alcohol, caffeine and wanking are distinguished as being addictions that are alright.