Fix hiding of parent members by @private in subclass.

Previously ClassFormatter removed all information about private
members, resulting in info about private members getting lost and
so @private in subclass not hiding public member in parent class.

Fixed by leaving all private members intact.

Additionally minor enhancement for test, so it actually tests if
private member in subclass hides public member of parent.  This
part of the code worked fine before too, just the test was not
really testing for it.