bench: Replace ::mempool globabl with test_setup.mempool

This is a refactor, since they are aliases for each other

src/bench/block_assemble.cpp

@@ -42,7 +42,7 @@ static void AssembleBlock(benchmark::State& state)
 
         for (const auto& txr : txs) {
             TxValidationState state;
-            bool ret{::AcceptToMemoryPool(::mempool, state, txr, nullptr /* plTxnReplaced */, false /* bypass_limits */, /* nAbsurdFee */ 0)};
+            bool ret{::AcceptToMemoryPool(*test_setup.m_node.mempool, state, txr, nullptr /* plTxnReplaced */, false /* bypass_limits */, /* nAbsurdFee */ 0)};
             assert(ret);
         }
     }