test/fuzz: Invoke LoadMempool via CChainState

Not only does this increase coverage, it is also more correct in that
when ::LoadMempool is called with a mempool and chainstate, it calls
AcceptToMemoryPool with just the chainstate.

AcceptToMemoryPool will then act on the chainstate's mempool via
CChainState::GetMempool, which may be different from the mempool
originally passed to ::LoadMempool. (In this fuzz test's case, it
definitely is different)

Also, move DummyChainstate to its own file since it's now used by the
validation_load_mempool fuzz test to replace CChainState's m_mempool.

src/validation.h

@@ -679,7 +679,7 @@ public:
     void CheckBlockIndex();
 
     /** Load the persisted mempool from disk */
-    void LoadMempool(const ArgsManager& args);
+    void LoadMempool(const ArgsManager& args, fsbridge::FopenFn mockable_fopen_function = fsbridge::fopen);
 
     /** Update the chain tip based on database information, i.e. CoinsTip()'s best block. */
     bool LoadChainTip() EXCLUSIVE_LOCKS_REQUIRED(cs_main);