Merge bitcoin#25073: test: Cleanup miner_tests

faa1552 test: Use dedicated mempool in TestBasicMining (MacroFake)
fafab38 test: Use dedicated mempool in TestPackageSelection (MacroFake)
fa4055d test: Use dedicated mempool in TestPrioritisedMining (MacroFake)
fa29218 test: Pass mempool reference to AssemblerForTest (MacroFake)

Pull request description:

  This cleans up the miner tests:

  * Removes duplicate/redundant and thus confusing chainparams object.
  * Uses a fresh mempool for each subtest instead of using the "global" one from the testing setup. This makes it easier to follow the tests in smaller scopes. Also it makes sure the mempool is truly cleared by reconstructing it. Finally, this removes calls to `clear`, see bitcoin#19909

ACKs for top commit:
  glozow:
    utACK faa1552

Tree-SHA512: ced1260f6ab70fba74b0fac7ff4fc7adfddcd2f3bee785249d2a4a9055ac253eff9090edbda7a17e72a71a81b56ff708d5ff64e1f57ebc7b7747d6c88fec51e3

Author: fanquake