Add testnet-seed.bchd.cash to seeder lists

Summary:
BCHD mainnet seeder was previously added in D4073
This also adds the testnet seeder.

Test Plan:
make check
Type nslookup testnet-seed.bchd.cash and check that it returns a list of peers.
Confirm with BCHD folks that they plan to keep this seeder running for a long time.

Reviewers: deadalnix, jasonbcox, zquestz, #bitcoin_abc

Reviewed By: jasonbcox, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D4308