test: Move BnB feerate sensitivity tests

Originally these tests verified that at a SelectCoins level that a
solution with fewer inputs gets preferred at high feerates, and a
solution with more inputs gets preferred at low feerates. This outcome
relies on the behavior of BnB, so we move these tests under the umbrella
of BnB tests.

Originally these tests relied on SFFO to work.

src/wallet/test/coinselection_tests.cpp

@@ -86,12 +86,12 @@ static std::string InputAmountsToString(const SelectionResult& selection)
     return "[" + util::Join(selection.GetInputSet(), " ", [](const auto& input){ return util::ToString(input->txout.nValue);}) + "]";
 }
 
-static void TestBnBSuccess(std::string test_title, std::vector<OutputGroup>& utxo_pool, const CAmount& selection_target, const std::vector<CAmount>& expected_input_amounts, const CoinSelectionParams& cs_params = default_cs_params)
+static void TestBnBSuccess(std::string test_title, std::vector<OutputGroup>& utxo_pool, const CAmount& selection_target, const std::vector<CAmount>& expected_input_amounts, const CoinSelectionParams& cs_params = default_cs_params, int custom_spending_vsize = 68)
 {
     SelectionResult expected_result(CAmount(0), SelectionAlgorithm::BNB);
     CAmount expected_amount = 0;
     for (CAmount input_amount : expected_input_amounts) {
-        OutputGroup group = MakeCoin(input_amount, true, cs_params);
+        OutputGroup group = MakeCoin(input_amount, true, cs_params, custom_spending_vsize);
         expected_amount += group.m_value;
         expected_result.AddInput(group);
     }
@@ -118,5 +118,28 @@ BOOST_AUTO_TEST_CASE(bnb_test)
     TestBnBSuccess("Select upper bound", utxo_pool, /*selection_target=*/4 * CENT - default_cs_params.m_cost_of_change, /*expected_input_amounts=*/{1 * CENT, 3 * CENT});
 }
 
+BOOST_AUTO_TEST_CASE(bnb_feerate_sensitivity_test)
+{
+    // Create sets of UTXOs with the same effective amounts at different feerates (but different absolute amounts)
+    std::vector<OutputGroup> low_feerate_pool; // 5 sat/vB (default, and lower than long_term_feerate of 10 sat/vB)
+    AddCoins(low_feerate_pool, {2 * CENT, 3 * CENT, 5 * CENT, 10 * CENT});
+    TestBnBSuccess("Select many inputs at low feerates", low_feerate_pool, /*selection_target=*/10 * CENT, /*expected_input_amounts=*/{2 * CENT, 3 * CENT, 5 * CENT});
+
+    CoinSelectionParams high_feerate_params = init_default_params();
+    high_feerate_params.m_effective_feerate = CFeeRate{25'000};
+    std::vector<OutputGroup> high_feerate_pool; // 25 sat/vB (greater than long_term_feerate of 10 sat/vB)
+    AddCoins(high_feerate_pool, {2 * CENT, 3 * CENT, 5 * CENT, 10 * CENT}, high_feerate_params);
+    TestBnBSuccess("Select one input at high feerates", high_feerate_pool, /*selection_target=*/10 * CENT, /*expected_input_amounts=*/{10 * CENT}, high_feerate_params);
+
+    // Add heavy inputs {6, 7} to existing {2, 3, 5, 10}
+    low_feerate_pool.push_back(MakeCoin(6 * CENT, true, default_cs_params, /*custom_spending_vsize=*/500));
+    low_feerate_pool.push_back(MakeCoin(7 * CENT, true, default_cs_params, /*custom_spending_vsize=*/500));
+    TestBnBSuccess("Prefer two heavy inputs over two light inputs at low feerates", low_feerate_pool, /*selection_target=*/13 * CENT, /*expected_input_amounts=*/{6 * CENT, 7 * CENT}, default_cs_params, /*custom_spending_vsize=*/500);
+
+    high_feerate_pool.push_back(MakeCoin(6 * CENT, true, high_feerate_params, /*custom_spending_vsize=*/500));
+    high_feerate_pool.push_back(MakeCoin(7 * CENT, true, high_feerate_params, /*custom_spending_vsize=*/500));
+    TestBnBSuccess("Prefer two light inputs over two heavy inputs at high feerates", high_feerate_pool, /*selection_target=*/13 * CENT, /*expected_input_amounts=*/{3 * CENT, 10 * CENT}, high_feerate_params);
+}
+
 BOOST_AUTO_TEST_SUITE_END()
 } // namespace wallet