What do you do when a horse is testing you?

I would start with teaching ground manners. It seems like she was allowed to be the leader and just needs to be retaught. I would suggest keeping her away from other horses for a while and letting her get used to you being part of her "herd". I would lunge her until she's tired and then ride her, that way she can't act up because she's so tired. I would look for a natural horsemanship trainer or a cowboy. They pretty much can get a horse to do anything. My old horse was so afraid of cows that he would rear and fall over on me, after bringing him to a cowboy he never was afraid again. That horse would also freak out when the other horses were brought out into the pasture and he was alone. After kicking out in his stall and tearing his foot so bad we had to give him stitches, he never freaked out when he was alone.

It sounds like this horse needs something more to do that just run around the arena. Get her involved with reining around cones, or barrels, take her out where you can get her going around trees, through a ditch over a log, down a lane across a field, etc...give her something to think about. If you are merely riding around in circles in the arena she is going to be bored and start making up her own little game, just for fun. The man that told you to one rein her is right, when you are out on the trail and the horse needs emergency stop, or at the very least some control, this method works great. Start with the spin, circle, then larger circles if you have the room and you are not in the middle of a road somewhere. Otherwise there needs to be some basic training on stopping.

If a horse is testing you never let him get away with anything always make him go back and repeat what he did until he gets it right never let him get away with anything and make sure he knows your the boos and your the one telling him what to do not the other way around. Turning her in circles was right but another way to stop them is to give definite pulls (not jerks) with the outside rein (the rein nearest to the wall.

Putting the horse into circles is the correct way to do it. and to keep the horses mind on you, just when the horse gets distracted make her go a different pace than the other horses and get her to do large circles or make her do a weaving pattern.

Hope i helped!