Avoid using test classes in main classes

This is a bit fishy but Maven supports it so we used it

Unfortunately it's officially not the case anymore in Eclipse 4.8+ (it was not exactly the case before either, it's just that there was no clean separation between test and main classes in Eclipse before) so we should ideally clean this up a bit.

On example is xwiki-platform-test-page which manipulate stuff like MockitoOldcore and StubVelocityManager.