Errorprone separate testing project

Hello guys,

In our current work, we should develop several different BugCheckers for Errorprone. So, I've created a separate project from error_prone_core containing some of Errorprone test core classes.

I think that it's better because other developers can create tests out of error_prone_core. In our case, for example, we should use errorprone with some of our BugCheckers as a plugin, and it's better if we distribute only a simple "errorprone-testing" project instead of tell to developers "hey, clone this repos and create bugcheckers inside of it".

What do you think guys? If you want, I can start to work on this errorprone separate test project.