bench/rpc_mempool: Create ChainTestingSetup, use its CTxMemPool

This is correct because:

- The ChainTestingSetup is constructed before the call to bench.run(...)
- All the runs are performed on the same mempool

src/bench/rpc_mempool.cpp

@@ -3,7 +3,9 @@
 // file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
 #include <bench/bench.h>
+#include <chainparamsbase.h>
 #include <rpc/mempool.h>
+#include <test/util/setup_common.h>
 #include <txmempool.h>
 
 #include <univalue.h>
@@ -17,7 +19,8 @@ static void AddTx(const CTransactionRef& tx, const CAmount& fee, CTxMemPool& poo
 
 static void RpcMempool(benchmark::Bench& bench)
 {
-    CTxMemPool pool;
+    const auto testing_setup = MakeNoLogFileContext<const ChainTestingSetup>(CBaseChainParams::MAIN);
+    CTxMemPool& pool = *Assert(testing_setup->m_node.mempool);
     LOCK2(cs_main, pool.cs);
 
     for (int i = 0; i < 1000; ++i) {