As suggested in this message (http://www.tunnelbroker.net/forums/index.php?topic=555.msg15752#msg15752), it should be possible to resolve IPv6 address using IPv6 only, i.e.:
dig -6 aaaa user.domain
should succeed.
There could also be a test the other way around like this one: http://ds.v6ns.test-ipv6.com/
That one is of course much easier to pass (so easy I might have forgotten if it was even part of the certification in the first place).