tests: Add failing tests for GObject sub-class doc-strings

Add tests for sub-class docstrings set with either doc-string
syntax and by setting the __doc__ attribute directly.

Co-Authored-By: Simon Feltman <sfeltman@src.gnome.org>

https://bugzilla.gnome.org/show_bug.cgi?id=731452
Don't mask GObject sub-class doc strings in meta-class

If a class has a __doc__ attribute explicitly set, always return it.
Only generate doc strings for classes coming from  gi.repository
or gi.overrides.

Co-Authored-By: Simon Feltman <sfeltman@src.gnome.org>

https://bugzilla.gnome.org/show_bug.cgi?id=731452

https://bugzilla.gnome.org/show_bug.cgi?id=734926