Add mock DNS records for tests

[nodech]

The tests for the claims and anything related to it take too much time, because the requests depend on the response from the actual servers using stub resolver.

• It's performance depends on the stub server performance (e.g. 8.8.8.8 in bns)
• It depends on the DNS Records in those names (e.g. cloudflare) to have proper DNS records.

Ideally, we would have ability to mock the whole server. Proper place is probably the bns for this. (e.g. chjj/bns#34)

Alternatively, restructure Ownership/DNSProof in the ownership to have ability to mock the prov/verify for both the ownership and the proof.