Add constants to dns.h

Summary: Moves some constants from seeder/dns_tests.cpp to dns.h.

Test Plan:

ninja
ninja check-bitcoin-seeder

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D5459