Hi.
I've been doing an asthma trial that requires a lot of methacholine challenges to be done. I don't think there's any way of faking the test and getting a better result. If they take blood... or urine samples I'm sure they have a way of detecting a bronchodialator or any other asthma meds taken recently.
All you can do is try your best. Make sure you have 15 minutes of rest before performing the test. And keep active!
They say people who suffer from asthma suffer a lot less if they're regularly active compared to people who don't exercise.
Don't let it get you down. I know it sucks, but I think you have a good chance as a young male to outgrow this in the long run anyways.
Sorry, and good luck!