a horse will find a way to injure itself, you could put it in a padded room and it would still do it.
I think the people that have trouble with horses testing electric fences are those that don't have a good fencer and a good fence.
If you have a good fence that isn't grounded out by weeds/brush ect... a good fencer, and provide the horse with space, feed, water they won't likely test the fence very often. Once they learn the fence and know it's going to hurt they won't have problems with a straight high tensil (bare wire) fence
Just my .02