All testing shall be done with equipment appropriate to the needs and abilities of the team.
However, a correctly handled and trained dog will not back out of his collar. In order to back out of his collar he would have to tighten the leash.
Frankly, if a "service dog" cannot pass a public access test on a plain buckle collar something is wrong. I'm not talking about working in public, when special equipment might be needed. During a test of JUST the public access skills, a dog should be able to function with the most basic of equipment.
Remember that the function of a buckle collar on a trained dog is to provide a place to attach the ID tags, and a a place to attach the leash for compliance with local leash laws. On a trained dog the collar isn't needed for control because the dog is obedient. Why is this important? Because equipment fails, collars and leashes break, leashes are dropped, and handlers fall down. If equipment is needed to control the dog, then control will be lost in any of these events.
A service dog should be worked on leash, in compliance with local leash laws, but he should be
capable of working buck naked and off-leash. Of all the possible jobs a working dog might have, service work is the most unpredictable.