refactor: make MempoolMutex() public

for use in the following unittests.

Author: jamesob

src/validation.h

@@ -748,6 +748,12 @@ class Chainstate
 
     std::string ToString() EXCLUSIVE_LOCKS_REQUIRED(::cs_main);
 
+    //! Indirection necessary to make lock annotations work with an optional mempool.
+    RecursiveMutex* MempoolMutex() const LOCK_RETURNED(m_mempool->cs)
+    {
+        return m_mempool ? &m_mempool->cs : nullptr;
+    }
+
 private:
     bool ActivateBestChainStep(BlockValidationState& state, CBlockIndex* pindexMostWork, const std::shared_ptr<const CBlock>& pblock, bool& fInvalidFound, ConnectTrace& connectTrace) EXCLUSIVE_LOCKS_REQUIRED(cs_main, m_mempool->cs);
     bool ConnectTip(BlockValidationState& state, CBlockIndex* pindexNew, const std::shared_ptr<const CBlock>& pblock, ConnectTrace& connectTrace, DisconnectedBlockTransactions& disconnectpool) EXCLUSIVE_LOCKS_REQUIRED(cs_main, m_mempool->cs);
@@ -761,12 +767,6 @@ class Chainstate
     void CheckForkWarningConditions() EXCLUSIVE_LOCKS_REQUIRED(cs_main);
     void InvalidChainFound(CBlockIndex* pindexNew) EXCLUSIVE_LOCKS_REQUIRED(cs_main);
 
-    //! Indirection necessary to make lock annotations work with an optional mempool.
-    RecursiveMutex* MempoolMutex() const LOCK_RETURNED(m_mempool->cs)
-    {
-        return m_mempool ? &m_mempool->cs : nullptr;
-    }
-
     /**
      * Make mempool consistent after a reorg, by re-adding or recursively erasing
      * disconnected block transactions from the mempool, and also removing any