add TestTagManager::test_destructor

This test needs MockTag and MockTagManager classes to work properly :
  - The base TagManager class doesn't provides a addTag method, which
is needed to add the mocked Tags.
  - The base Tag class doesn't provieds a way to know if the Tag has been well
deleted.

Modification of TagManager and Tag destructors to virtual to handle subclasses
(mocks) deletion.
Modification of TagManager's Tag vector scope to protected to give an access
to its subclasses (mocks)
change MockItem constructor boolean pointer to reference