Best practive for JUnit package naming?

Typically I put the test classes in the same package, but under a different source root. Aside from anything else, this allows you to test members (and indeed classes) which have default visibility. Sometimes I'll even make methods which would otherwise be private, package-level to make testing easier, knowing that it will only have limited impact. This very much depends on the context in which you're developing though. (Most of my code is written in a trusted environment; I want to limit access for elegance, but I'm not worried that anyone will actively try to use a method they shouldn't. And I use an annotation to indicate the intention.)

You definitely want an easy way of separating out your test classes from your production classes, both for deployment purposes and to let you concentrate on "just the prod code" when appropriate. But I don't see any disadvantage in using the same package declaration for both tests and production code.