tests: Add tests for overriding vfunc implementations
tests: Add tests for overriding vfunc implementations

Add tests for overriding vfuncs for both single inheritance
and multiple inheritance with an interface (currently failing).

https://bugzilla.gnome.org/show_bug.cgi?id=700092

sfeltman/pygobject,davibe/pygobject,GNOME/pygobject,pexip/pygobject,GNOME/pygobject,sfeltman/pygobject,davibe/pygobject,GNOME/pygobject,pexip/pygobject,davibe/pygobject,sfeltman/pygobject,pexip/pygobject,davibe/pygobject