For `MempoolCheck`, it seems that the intention was to have a mempool with `check_ratio = 1` (see `-checkmempool=1`):

https://github.com/bitcoin/bitcoin/blob/e66630cc87c017f40ec29f6c1edf2ed5a286e49d/src/bench/mempool_stress.cpp#L102-L116

However in the subsequent line in the above snippet, a `CTxMemPool` gets constructed with no arguments, which means that `check_ratio` will default to 0:

https://github.com/bitcoin/bitcoin/blob/e66630cc87c017f40ec29f6c1edf2ed5a286e49d/src/txmempool.h#L571

Manually specifying `check_ratio` to 1 (according to the original intention) and running the `MempoolCheck` benchmark results in an assertion error:

```
diff --git a/src/bench/mempool_stress.cpp b/src/bench/mempool_stress.cpp
index afa4618e1b..32cdb70539 100644
--- a/src/bench/mempool_stress.cpp
+++ b/src/bench/mempool_stress.cpp
@@ -105,7 +105,7 @@ static void MempoolCheck(benchmark::Bench& bench)
     const int childTxs = bench.complexityN() > 1 ? static_cast<int>(bench.complexityN()) : 2000;
     const std::vector<CTransactionRef> ordered_coins = CreateOrderedCoins(det_rand, childTxs, /* min_ancestors */ 5);
     const auto testing_setup = MakeNoLogFileContext<const TestingSetup>(CBaseChainParams::MAIN, {"-checkmempool=1"});
-    CTxMemPool pool;
+    CTxMemPool pool{nullptr, 1};
     LOCK2(cs_main, pool.cs);
     const CCoinsViewCache& coins_tip = testing_setup.get()->m_node.chainman->ActiveChainstate().CoinsTip();
     for (auto& tx : ordered_coins) AddTx(tx, pool);
```

```sh
$ ./src/bench/bench_bitcoin -filter='MempoolCheck'
...
bench_bitcoin: txmempool.cpp:759: void CTxMemPool::check(const CCoinsViewCache &, int64_t) const: Assertion `mempoolDuplicate.HaveCoin(txin.prevout)' failed.
```

Aside from this problem, we should also probably just re-use the `CTxMemPool` in `TestingSetup`.

Ping @glozow
for (size_t ancestor = 0; ancestor < n_ancestors && !available_coins.empty(); ++ancestor){
            size_t idx = det_rand.randrange(available_coins.size());
            Available coin = available_coins[idx];
            uint256 hash = coin.ref->GetHash();
            
            size_t n_to_take = det_rand.randrange(2) == 0 ?
                               min_ancestors :
                               min_ancestors + det_rand.randrange(coin.ref->vout.size() - coin.vin_left);
            for (size_t i = 0; i < n_to_take; ++i) {
                tx.vin.emplace_back();
                tx.vin.back().prevout = COutPoint(hash, coin.vin_left++);
                tx.vin.back().scriptSig = CScript() << coin.tx_count;
                tx.vin.back().scriptWitness.stack.push_back(CScriptNum(coin.tx_count).getvch());
            }
            if (coin.vin_left == coin.ref->vin.size()) {
                coin = available_coins.back();
                available_coins.pop_back();
            }
            tx.vout.resize(det_rand.randrange(10)+2);
            for (auto& out : tx.vout) {
                out.scriptPubKey = CScript() << CScriptNum(tx_counter) << OP_EQUAL;
                out.nValue = 10 * COIN;
            }
        }
        ordered_coins.emplace_back(MakeTransactionRef(tx));
        available_coins.emplace_back(ordered_coins.back(), tx_counter++);
    }
    return ordered_coins;
}
static void ComplexMemPool(benchmark::Bench& bench)