tests/qapi-schema: Rewrite simple union TestIfUnion to be flat

tests/qapi-schema: Rewrite simple union TestIfUnion to be flat

Simple unions predate flat unions.  Having both complicates the QAPI
schema language and the QAPI generator.  We haven't been using simple
unions in new code for a long time, because they are less flexible and
somewhat awkward on the wire.

To prepare for their removal, rewrite TestIfUnion to be flat.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20210917143134.412106-18-armbru@redhat.com>