We were previously preventing implementations of an interface from
specifying G_PARAM_CONSTRUCT for a property of that interface if the
interface didn't specify it itself (or was readonly).
This is something that should only interest the implementation, so we
remove this restriction.
This allows 6 new possible override scenarios:
- writable -> writable/construct
- writable -> readwrite/construct
- readwrite -> readwrite/construct
- writable/construct-only -> writable/construct
- writable/construct-only -> readwrite/construct
- readwrite/construct-only -> readwrite/construct
and we update the testcase to reflect this.
https://bugzilla.gnome.org/show_bug.cgi?id=666616