Make junit categories work better when running tests across multiple projects

Make junit categories work better when running tests across multiple projects. Even for a single project, this is probably closer to the desired behavior when referencing a class that does not exist

@jroper this new feature seems to be working pretty well for us! Wanted to share this one small improvement
This is saying if one of the categories can't be found, none of them should be returned.  If we're going to ignore class not found, it should be per category.