Adjust cdef_members_T517 tests to reflect visibility for struct attributes.

Adjust cdef_members_T517 tests to reflect visibility for struct attributes.

Struct attributes can have private or readonly, but not public, Python
visibility.

Readonly structs are currently converted to dicts on the fly for each
call getting the struct attribute value.  The dict-generation code is
currently broken for C++ (error: declaration does not declare
anything), so tests/run/cdef_members_T517.pyx currently fails a C++
test.  However, we hope to soon replace it with full blown Python
classes representing structs/unions/enums which will work for both C
and C++.