miner_tests: pass chainman and active_chainstate to helper functions

Summary:
This removes usage of global chainstate for Bitcoin ABC specific code.
Also make the functions static.

Test Plan: ninja check

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D11650