csharp: Fix wrapping of private classes

csharp: Fix wrapping of private classes

Summary:
Sometimes, valid Eo objects of private classes can be returned from
methods. Currently we try to wrap in a minimal `Efl.Object` instance,
but as it is an abstract class, we can't instantiate directly.

This commits adds a dummy `Efl.ObjectRealized` to be instantiated when
wrapping such classes alongside a test case.

Fixes: T8258

Reviewers: felipealmeida, brunobelo, segfaultxavi, Jaehyun_Cho

Reviewed By: brunobelo

Subscribers: cedric, #reviewers, #committers

Tags: #efl

Differential Revision: https://phab.enlightenment.org/D10101