tests/qapi-schema: Wean off UserDefListUnion

Command boxed-union uses simple union UserDefListUnion to cover
unions.  Use UserDefFlatUnion instead.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20210917143134.412106-16-armbru@redhat.com>
3 files changed