Cleanup Bitcoin Cash user agents in seeder tests

Summary:
Coupled with D10104 and D10105, BCH references in the seeder should be cleaned up
and rebranded to eCash (or Bitcoin ABC as determined by the node).

Test Plan:

ninja check-seeder

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10106