tests: Add proof-of-work fuzzing harness

Summary:
Backport of core PR18407.

Depends on D8981.

Test Plan:

ninja bitcoin-fuzzers
./test/test/fuzz/pow <path_to_corpus>

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D8984