I know that ez test has had booths set up before.
The problem is that people will think that you are advocating the use of MDMA. And regardless of the fact that you are at a rave, it is still looked down upon and illegal.
Your intent, while admirable, does not make it legal.
If you set it up outside, you're doing nothing but making the job of security easier...they see everyone walking away from your booth and flag them for search.
If you have it inside...well you're gonna lose a bunch of money, because you'll need more than one kit, and you gotta drop a pretty penny on those booth rentals...
I'll be honest, it's a great idea. But you'd be better off taking your test kit in and offering your services that way.