modelbuilder: AExternInitPropdef implicit name is `init`

because `new X` is `new X.init`

Signed-off-by: Jean Privat <jean@pryen.org>
nitg: NOT YET IMPLEMENTED on AExternInitPropdef

Signed-off-by: Jean Privat <jean@pryen.org>
tests: remove most ffi-related skips

NOT YET IMPLEMENTS automatically skips them.

Signed-off-by: Jean Privat <jean@pryen.org>