Test headers sync using minchainwork threshold

Summary:
This is a partial backport of core#25717
https://github.com/bitcoin/bitcoin/pull/25717/commits/150a5486db50ff77c91765392149000029c8a309

Depends on D15165

Test Plan: ninja check-functional

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D15166