The domain mooo.com is hosted on ns{1,2,3,4}.afraid.org, of those only ns1 has a AAAA record. If you still get the message saying "Completed: The NS have AAAA records", then that means the test is able to find that AAAA record.
There is a couple of problems though. In order to find that AAAA record in the first place, you first have to lookup ns1.afraid.org, but the authoritative DNS servers for afraid.org don't have AAAA records. This means even though ns1.afraid.org has IPv6 support, that support is essentially useless as an IPv6 only DNS server won't be able to reach it. Additionally it is only ns1.afraid.org which has a AAAA record. ns{2,3,4}.afraid.org are IPv4 only.
So the IPv6 support on afraid.org is almost nonexistent. However the problems I mention would happen before looking up the AAAA record. The test did succeed in finding the AAAA record. Once it has found the AAAA record it would just need to query the IPv6 address in question, I tested that part, and it works.
So if the error message still looks the same, then I think the test is broken or the error message is misleading. In either case I think a ticket would be appropriate.
When I took the test I did run into a similar issue where I was supposed to use the parent domain (the top most domain hosted on that NS), but the error message I got about that was very misleading. Though the misleading error message I got was different, it may still be the same issue. Did you actually try to enter just mooo.com in the test instead of the subdomain learningipv6.mooo.com?