tests/qapi-schema: Cover forward reference to sub-module

The forward reference from the main module to the sub-module works
fine, except for an issue visible in qapi-schema-test.out: the array
type wrapped around the forward reference ends up in the main module,
not the sub-module.  The next commit will explain why that's bad, and
fix it.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20190301154051.23317-7-armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>