Rename nullifier caches and maps to indicate sprout nullifiers

src/gtest/test_mempool.cpp

@@ -56,7 +56,7 @@ public:
                     const uint256 &hashBlock,
                     const uint256 &hashAnchor,
                     CAnchorsMap &mapAnchors,
-                    CNullifiersMap &mapNullifiers,
+                    CNullifiersMap &mapSproutNullifiers,
                     CNullifiersMap &mapSaplingNullifiers) {
         return false;
     }