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/pow.cpp

@@ -134,7 +134,18 @@ bool PermittedDifficultyTransition(const Consensus::Params& params, int64_t heig
     return true;
 }
 
+// Bypasses the actual proof of work check during fuzz testing with a simplified validation checking whether
+// the most signficant bit of the last byte of the hash is set.
 bool CheckProofOfWork(uint256 hash, unsigned int nBits, const Consensus::Params& params)
+{
+#ifdef FUZZING_BUILD_MODE_UNSAFE_FOR_PRODUCTION
+    return (hash.data()[31] & 0x80) == 0;
+#else
+    return CheckProofOfWorkImpl(hash, nBits, params);
+#endif
+}
+
+bool CheckProofOfWorkImpl(uint256 hash, unsigned int nBits, const Consensus::Params& params)
 {
     bool fNegative;
     bool fOverflow;