test-qobject-input-visitor: Wean off UserDefListUnion

test-qobject-input-visitor: Wean off UserDefListUnion

The test_visitor_in_list_union_FOO() use simple union UserDefListUnion
to cover lists of builtin types.  Rewrite as
test_visitor_in_list_struct(), using struct ArrayStruct and a lot less
code.

test_visitor_in_fail_union_list() uses UserDefListUnion to cover
"variant members don't match the discriminator value".  Cover that in
test_visitor_in_fail_union_flat() instead, and drop
test_visitor_in_fail_union_list().  Appropriating the former for this
purpose is okay, because it actually failed due to missing
discriminator, which is still covered by
test_visitor_in_fail_union_flat_no_discrim().

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20210917143134.412106-13-armbru@redhat.com>