test: Fix intermittent sync_blocks failures

Summary:
Backport of core PR18873.

The version check is not ported as we don't expect user to run the tests
against an older version of bitcoind.

Test Plan:

ninja check-functional

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D8337