Revert "Remove testnet-only difficulty rules"

This reverts commit 333ea3c426.

src/pow.cpp

@@ -24,6 +24,23 @@ unsigned int GetNextWorkRequired(const CBlockIndex* pindexLast, const CBlockHead
     if (pindexLast == NULL)
         return nProofOfWorkLimit;
 
+    const CBlockIndex* pindexBits = pindexLast;
+    {
+        if (params.fPowAllowMinDifficultyBlocks)
+        {
+            // Special difficulty rule for testnet:
+            // If the new block's timestamp is more than 2* 2.5 minutes
+            // then allow mining of a min-difficulty block.
+            if (pblock->GetBlockTime() > pindexLast->GetBlockTime() + params.nPowTargetSpacing*2)
+                return nProofOfWorkLimit;
+            else {
+                // Get the last non-min-difficulty (or at worst the genesis difficulty)
+                while (pindexBits->pprev && pindexBits->nBits == nProofOfWorkLimit)
+                    pindexBits = pindexBits->pprev;
+            }
+        }
+    }
+
     // Find the first block in the averaging interval
     const CBlockIndex* pindexFirst = pindexLast;
     arith_uint256 bnTot {0};