tests: Rename UserDefNativeListUnion to UserDefListUnion

The lists in UserDefNativeListUnion aren't "native", they're lists of
built-in types.  The next commit will add a list of a user-defined
type.  Drop "Native", and adjust the tests using the type.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20190301154051.23317-6-armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>