The short answer is a good pop of the crop behind your leg to tell him to get off that leg and move away from the pressure. The problem is, if you're not a rider with a secure seat, the result might be you on the ground.
Venturing from lesson horses to testing horses is always a little "not fun" at first. Just be very clear with your aids. Start at the walk, pick an object on the other side of the arena and ride to it. Stay off the rail as much as you can. You don't want the horse to go into auto-pilot mode. If you pick objects and ride to them, it keeps his attention on you because he doesn't know what's coming next. If he starts to wander off course, two arms/two legs, use all your aids to keep him straight. Also try riding circles with a ground pole, so you have something to focus on riding over. When all is working at the walk, do it at the trot and then finally canter.
Horse's have a very good 6th sense about how determined we are as riders. Once they understand that you plan on be very clear in your riding, they will respond in a more positive way. Sounds like yours is just having a little fun. Once she understands you mean what you say, so to speak, she'll come around.
You just have to see your distance...you don't have to like it.