sounds like your Sensei already established the protocol. after that, it's the guy choosing to be a ______, probably out of some ego trip or whatever.
"If it's understood between nage and uke that you are helping each other train by giving a bit of resistance to help eachother fine tune their techniques, I think that's fine--but sometimes I find people who deliberately go around trying to resist and test people and it causes injuries."
also sounds like you understand well enough what the proper place of resistance should be..
to me, the bottom line is, are people being nice and friendly, or are they going out of their way to be confrontational and competitive etc?... for me the latter goes against aikido training principles, whereas the former can indeed foster an environment where people can test and learn and grow together..
you will find that this issue is a huge one for Aikido, and you will keep coming back to it..