gobject: forbid finalization-during-construction

If a constructor() implementation created an object but then unreffed
it rather than returning it, that object would get left on the
construction_objects list, which would cause problems later when that
memory location got reused by another object.

"Fix" this by making it fail intentionally, and add a test for it (and
for the normal, working singleton case).

https://bugzilla.gnome.org/show_bug.cgi?id=661576