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
#include <consensus/validation.h>
#include <key.h>
#include <script/sign.h>
#include <script/signingprovider.h>
#include <script/standard.h>
#include <test/util/setup_common.h>
#include <txmempool.h>
#include <validation.h>
#include <boost/test/unit_test.hpp>
struct Dersig100Setup : public TestChain100Setup {
    Dersig100Setup()
        : TestChain100Setup{{"-testactivationheight=dersig@102"}} {}
};
bool CheckInputScripts(const CTransaction& tx, TxValidationState& state,
                       const CCoinsViewCache& inputs, unsigned int flags, bool cacheSigStore,
                       bool cacheFullScriptStore, PrecomputedTransactionData& txdata,
                       std::vector<CScriptCheck>* pvChecks) EXCLUSIVE_LOCKS_REQUIRED(cs_main);
BOOST_AUTO_TEST_SUITE(txvalidationcache_tests)
BOOST_FIXTURE_TEST_CASE(tx_mempool_block_doublespend, Dersig100Setup)