Add FUZZING_BUILD_MODE_UNSAFE_FOR_PRODUCTION in PoW check

To avoid PoW being a blocker for fuzz tests,
`FUZZING_BUILD_MODE_UNSAFE_FOR_PRODUCTION` is used in fuzz builds to
bypass the actual PoW validation in `CheckProofOfWork`. It's
replaced with a check on the last byte of the hash, which allows the
fuzzer to quickly generate (in)valid blocks by checking a single bit,
rather than performing the full PoW computation.

If PoW is the target of a fuzz test, then it should call
`CheckProofOfWorkImpl`.

src/test/fuzz/pow.cpp

@@ -80,7 +80,7 @@ FUZZ_TARGET(pow, .init = initialize_pow)
         {
             const std::optional<uint256> hash = ConsumeDeserializable<uint256>(fuzzed_data_provider);
             if (hash) {
-                (void)CheckProofOfWork(*hash, fuzzed_data_provider.ConsumeIntegral<unsigned int>(), consensus_params);
+                (void)CheckProofOfWorkImpl(*hash, fuzzed_data_provider.ConsumeIntegral<unsigned int>(), consensus_params);
             }
         }
     }