The unversioned parent test with update_fields is an expectedFailure

Until Django #20272 is fixed, when the VersionField is excluded from
update_fields and there are no other fields to update on the versioned
class, checking can not happen because _do_update isn't called.
Clarify behavior with deferred version fields

It doesn't make sense to defer the VersionField and then save, so throw
an exception when it happens.