Added abc-sync-chain test to ensure node doesn't seize while syncing many blocks

Summary:
As part of investigating test flakiness in abc-parkedchain.py, I noticed a
severe lack of test coverage for nodes syncing many out-of-order blocks, both in and
out of IBD. This test provides that explicit coverage so that further development
on parking blocks does not cause nodes to periodically seize while syncing.

Test Plan:
Run multiple times:
for i in {1..100}; do ./test_runner.py abc-sync-chain; done

Reviewers: deadalnix, Fabien, #bitcoin_abc

Reviewed By: Fabien, #bitcoin_abc

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2188