Good question, here is a bad answer, no there is no place to get this specific type training, thats why most testing guys come from the military, lots of Nany Nukes and Army prime power guys in the field. Or you stumble across it by learning in the field, truth is most companies that do testing just steal the guys from other testing companies.
NETA (International Electrical Testing Association)
www.netaworld.org is the only place to get certified in the electrical testing field, I sit on the training commitee and we are discussing this very problem, developing courses to bring new people into the industry, there were only a few places to get this type of training and I was one of them until I retired from training a few months ago. AVO and Shermco are good places to start, but the courses are specific to breakers, transformers, etc. To cover all the bases you would be there a year or so.
A good testing guy has to have a very strong electrical theory knowledge, like EE level stuff, not trade school stuff, with a strong theory background everything else can be learned on the job if you can get in the door.