How about doing a DNS lookup using IPv6.
In order to do this, one would need a DNS zone that is deliberately NOT delegated to, on a server that only accepts IPv6 requests. Then the testee would fetch something from the server, and enter that as evidence of test completion.
I suggest that it be a TXT record, using a domain name like <username>.hurricane-electric-certification-test. and that the value be a nonce, followed by a hash of the nonce, username, and a secret shared between the dns server and the test server. This would prevent sharing answers, or taking too long to enter the answer.