tests/qapi-schema: Prepare for simple union UserDefListUnion removal

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, simple union UserDefListUnion has to go.
It is used to cover arrays.  The next few commits will eliminate its
uses, and then it gets deleted.  As a first step, provide struct
ArrayStruct for the tests to be rewritten.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20210917143134.412106-12-armbru@redhat.com>