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
const std::vector<CMutableTransaction>& txns,
    const CScript& scriptPubKey,
    CChainState* chainstate)
{
    if (!chainstate) {
        chainstate = &Assert(m_node.chainman)->ActiveChainstate();
    }
    const CBlock block = this->CreateBlock(txns, scriptPubKey, *chainstate);
    std::shared_ptr<const CBlock> shared_pblock = std::make_shared<const CBlock>(block);
    Assert(m_node.chainman)->ProcessNewBlock(shared_pblock, true, nullptr);
    return block;
}
CMutableTransaction TestChain100Setup::CreateValidMempoolTransaction(CTransactionRef input_transaction,
                                                                     int input_vout,
                                                                     int input_height,
                                                                     CKey input_signing_key,
                                                                     CScript output_destination,
                                                                     CAmount output_amount,
                                                                     bool submit)
{
    
    CMutableTransaction mempool_txn;
    
    COutPoint outpoint_to_spend(input_transaction->GetHash(), input_vout);
    CTxIn input(outpoint_to_spend);
    mempool_txn.vin.push_back(input);