tests/qapi-schema: Rename redefined-builtin to redefined-predefined

The previous commit changed this test to clash with a predefined enum
type, not a built-in type.  Adjust its name.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20210323094025.3569441-16-armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>