[tests] Readability refactors for miner_tests

Summary: This is a refactor only (no behavior change) to the tests to improve readability and thread safety assertions but also make the coming preconsensus mining tests easier to review.

Test Plan:

ninja check-bitcoin-miner_tests

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D18374