Rename seeder_tests to p2p_messaging_tests

Summary:
seeder_tests is vague and does not properly describe what is being
tested. p2p_messaging_tests better describes what is being
tested.

Test Plan:

make check
ninja check
ninja check-bitcoin-seeder

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, jasonbcox

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, jasonbcox

Subscribers: jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D5401