[prep/util] help MockMempoolMinFee handle more precise feerates

Use a virtual size of 1000 to keep precision when using a feerate
(which is rounded to the nearest satoshi per kvb) that isn't just an
integer.

src/test/util/setup_common.cpp

@@ -40,6 +40,7 @@
 #include <test/util/coverage.h>
 #include <test/util/net.h>
 #include <test/util/random.h>
+#include <test/util/transaction_utils.h>
 #include <test/util/txmempool.h>
 #include <txdb.h>
 #include <txmempool.h>
@@ -586,6 +587,9 @@ void TestChain100Setup::MockMempoolMinFee(const CFeeRate& target_feerate)
     CMutableTransaction mtx = CMutableTransaction();
     mtx.vin.emplace_back(COutPoint{Txid::FromUint256(m_rng.rand256()), 0});
     mtx.vout.emplace_back(1 * COIN, GetScriptForDestination(WitnessV0ScriptHash(CScript() << OP_TRUE)));
+    // Set a large size so that the fee evaluated at target_feerate (which is usually in sats/kvB) is an integer.
+    // Otherwise, GetMinFee() may end up slightly different from target_feerate.
+    BulkTransaction(mtx, 4000);
     const auto tx{MakeTransactionRef(mtx)};
     LockPoints lp;
     // The new mempool min feerate is equal to the removed package's feerate + incremental feerate.