Add testnet DNS seed of Andreas Schildbach.

It runs sipas crawler, but rather than using its custom nameserver implementation it serves a generated zonefile via bind9. The zone always contains 25 IPv4 and 25 IPv6 peers. FWIW, the zone is secured using DNSSEC.
Merge pull request #4862

45a4baf Add testnet DNS seed of Andreas Schildbach. (Andreas Schildbach)