0cc7dd74e0af735dddf7e786f4ed136c382a4ad5 test: add unittests for CheckProofOfWork (soroosh-sdi)
Pull request description:
following situations are covered:
- negative target
- overflow target
- target easier then powLimit
- invalid hash (hash > target)
Backport of Core [[https://github.com/bitcoin/bitcoin/pull/16931 | PR16931]]