fuzz: try to add more code coverage for mempool fuzzing

Including test coverage for mempool eviction and expiry

src/txmempool.cpp

@@ -418,6 +418,7 @@ void CTxMemPool::check(const CCoinsViewCache& active_coins_tip, int64_t spendhei
     uint64_t innerUsage = 0;
 
     assert(!m_txgraph->IsOversized(TxGraph::Level::MAIN));
+    m_txgraph->SanityCheck();
 
     CCoinsViewCache mempoolDuplicate(const_cast<CCoinsViewCache*>(&active_coins_tip));