[test] allow passing test checkpoints to ContextualCheckBlockHeader

Summary:
This enables testing the checkpoints without having to hack a Config and a CChainParams.
It is an alternative approach to D15802 and unbreaks the address sanitizer build that was broken by that diff.

Test Plan:
ninja all check-all

address sanitizer build

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D15825