Add unit test for HeadersSyncState

Summary:
This is a partial backport of core#25717
https://github.com/bitcoin/bitcoin/pull/25717/commits/0b6aa826b53470c9cc8ef4a153fa710dce80882f

Depends on D15164

Test Plan: ninja check

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D15165