tests/qapi-schema: Cover flat union types

The test demonstrates a generator bug: the generated struct
UserDefFlatUnion doesn't include members for the indirect base
UserDefZero.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>