Add some unit tests for write_name() for seeder

Summary:
Adds some unit tests for write_name() in seeder/dns.cpp. This
function, like parse_name() is critical for dns operations and should
have some tests to increase confidence in correct behavior.

Test Plan:

ninja check-bitcoin-seeder

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D5469