Merge bitcoin#31581: test: have miner_tests use Mining interface

0424968 test: use Mining interface in miner_tests (Sjors Provoost)

Pull request description:

  Needed for both bitcoin#31283 and bitcoin#31564.

  By using the Mining interface in `miner_tests.cpp` we increase its coverage in unit tests.

ACKs for top commit:
  achow101:
    ACK 0424968
  ryanofsky:
    Code review ACK 0424968, just minor suggested changes (renames, comments, BOOST_REQUIREs) since last review and some more extra clarifications and checks added to the CreateNewBlock_validity test. The CreateNewBlock_validity changes seem clear and easy to understand now.
  vasild:
    ACK 0424968
  tdb3:
    ACK 0424968

Tree-SHA512: 2761cb7555d759670e40d8f37b96a079f8e12a588ac43313b9e63c69afd478321515873a8896ea56784f0100dac4476b0c0e0ef8b5418f8aea24d9965cace4d4

Author: achow101