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
struct TestMemPoolEntryHelper
{
    
    CAmount nFee;
    int64_t nTime;
    unsigned int nHeight;
    bool spendsCoinbase;
    unsigned int sigOpCost;
    LockPoints lp;
    TestMemPoolEntryHelper() :
        nFee(0), nTime(0), nHeight(1),
        spendsCoinbase(false), sigOpCost(4) { }
    CTxMemPoolEntry FromTx(const CMutableTransaction& tx) const;
    CTxMemPoolEntry FromTx(const CTransactionRef& tx) const;
    
    TestMemPoolEntryHelper &Fee(CAmount _fee) { nFee = _fee; return *this; }
    TestMemPoolEntryHelper &Time(int64_t _time) { nTime = _time; return *this; }
    TestMemPoolEntryHelper &Height(unsigned int _height) { nHeight = _height; return *this; }
    TestMemPoolEntryHelper &SpendsCoinbase(bool _flag) { spendsCoinbase = _flag; return *this; }
    TestMemPoolEntryHelper &SigOpsCost(unsigned int _sigopsCost) { sigOpCost = _sigopsCost; return *this; }
};
CBlock getBlock13b8a();
std::ostream& operator<<(std::ostream& os, const uint256& num);
/**
 * BOOST_CHECK_EXCEPTION predicates to check the specific validation error.
 * Use as
 * BOOST_CHECK_EXCEPTION(code that throws, exception type, HasReason("foo"));
 */
class HasReason