Added .OnMessage<TMessage>(TMessage) to remove noise of forced message id during testing

- Added NServiceBus.Testing.Tests project to solution
- Fixed NServiceBus.Testing.Tests references and NUnit nuget package
- Added implementation for Handler<T>.OnMessage(T) for simplicity + test case

After messing around with pull requests, I finally made it :)
** Rectified pull request for pull request #862 and #863 (on develop branch) **