Add test-seeds CI configs

Summary:
This will allow us to run test-seeds in an automated way on a periodic basis so that we
can be notified when the number of reachable seed nodes drops below an expected threshold.
Thresholds were chosen somewhat arbitrarily based on my personal observations during releases
(typical values are >88% for mainnet and testnet, though testnet tends to vary a bit more).

Depends on D4724

Test Plan:

ABC_BUILD_NAME=check-seeds-mainnet ./build-configurations.sh
ABC_BUILD_NAME=check-seeds-testnet ./build-configurations.sh

Reviewers: #bitcoin_abc, deadalnix, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D4725