Improve testing for the difficulty adjustment algorithm.
Improve testing for the difficulty adjustment algorithm.

Summary: It now tests that we plateau at the minimal pow limit.

Test Plan:
  make check

Reviewers: kyuupichan, #bitcoin_abc, freetrader

Reviewed By: #bitcoin_abc, freetrader

Subscribers: freetrader

Differential Revision: https://reviews.bitcoinabc.org/D353

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