Move code in seeder_test.cpp close to where it is used

Summary: Move code to be closer to where it is actually used.

Test Plan:

ninja check-bitcoin-seeder

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D5402