A test to show you are addicted to opiates does not exist, and also your own experiences do not represent the experiences of every person that ever used drugs.
Yes, technically the tests are to show the opposite - that you are (not) opiate naive. Someone who can pass multiple closely spaced toxicology screens for heroin (i.e. opiates are not detected) is never going to be admitted to one of these maintenance programs due to the incredibly high risk of death by overdose.
A test to show you are addicted to opiates does not exist, and also your own experiences do not represent the experiences of every person that ever used drugs.