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>
diff --git a/tests/unit/test-qmp-cmds.c b/tests/unit/test-qmp-cmds.c
index 83efa39..83c9ef5 100644
--- a/tests/unit/test-qmp-cmds.c
+++ b/tests/unit/test-qmp-cmds.c
@@ -119,7 +119,7 @@
 {
 }
 
-void qmp_boxed_union(UserDefListUnion *arg, Error **errp)
+void qmp_boxed_union(UserDefFlatUnion *arg, Error **errp)
 {
 }