That is a good convention.
Sometimes you want to write unit tests for package-private classes and methods also. You won't be able to call them from a unit test class placed in another package.
There shouldn't be any confusion about having unit test classes in the same namespace as they shouldn't be in the class path when compiling or running the production code.
Here's an example of a small module with a public interface, a public factory class and two package-private implementation classes:
public interface Transmogrifier
public class TransmogrifierFactory
class MapTransmogrifier implements Transmogrifier
class ListTransmogrifier implements Transmogrifier
public class TransmogrifierFactoryTest
public class MapTransmogrifierTest
public class ListTransmogrifierTest
Hiding the implementations of the Transmogrifier interface could be a valid design choice. Perhaps it's the responsibility of the factory class to choose the implementation.
Since the implementations are package-private, you need to place the unit test classes in the same package if you wish to test them directly. If you have your unit test classes in some other package, you only have direct access to the public interface and the factory class from your tests.