Fix a race condition with rpc ports in check-seeds
The RPC port from the first run of test-seeds is not guaranteed to be closed/cleaned up
by the OS by the time the second run on test-seeds occurs. Without this patch, check-seeds is flaky
and fails at random.
Run this a few times on CI too
Reviewers: #bitcoin_abc, Fabien
Reviewed By: #bitcoin_abc, Fabien
Differential Revision: https://reviews.bitcoinabc.org/D5268