Make use of the DNSSEC flag in the DNS test

[brendonj]

The DNS test can set and check DNSSEC header flags, but it doesn't do anything with them. It should print information about DNSSEC validation and also report it to the server.

[brendonj]

The AD bit is already being reported as part of the flags.

The only obvious thing left is to actually perform the validation ourselves and see if it agrees with what the server reports. Is this something useful we want to do?