validation: add ResetChainstates()

Necessary for the following test commit.

src/validation.h

@@ -1051,6 +1051,8 @@ public:
     //! snapshot that is in the process of being validated.
     bool DetectSnapshotChainstate(CTxMemPool* mempool) EXCLUSIVE_LOCKS_REQUIRED(::cs_main);
 
+    void ResetChainstates() EXCLUSIVE_LOCKS_REQUIRED(::cs_main);
+
     //! Switch the active chainstate to one based on a UTXO snapshot that was loaded
     //! previously.
     Chainstate& ActivateExistingSnapshot(CTxMemPool* mempool, uint256 base_blockhash)