Merge bitcoin/bitcoin#25562: test: add tests for negative waste during coin selection

98ea43d test: add tests for negative waste during coin selection (ishaanam)

Pull request description:

  #25495 mentions that waste can be negative when the current feerate is less than the long term feerate. There are currently no waste tests for negative waste, so this PR adds two of them.

ACKs for top commit:
  achow101:
    ACK 98ea43d
  glozow:
    light code review ACK 98ea43d, good to have tests for negative waste

Tree-SHA512: d194d370f1257975959d3c601fea9f82c30c1aabc3e8bedc997c62659283fe681cc527e59df1a0187b3c91e8067c60374dd5ce0237561bd882edafe6a575a9b9

Author: achow101

src/wallet/test/coinselector_tests.cpp

@@ -867,7 +867,23 @@ BOOST_AUTO_TEST_CASE(waste_test)
     const CAmount new_target{in_amt - fee * 2 - fee_diff * 2};
     add_coin(1 * COIN, 1, selection, fee, fee + fee_diff);
     add_coin(2 * COIN, 2, selection, fee, fee + fee_diff);
-    BOOST_CHECK_EQUAL(0, GetSelectionWaste(selection, /* change cost */ 0, new_target));
+    BOOST_CHECK_EQUAL(0, GetSelectionWaste(selection, /*change_cost=*/ 0, new_target));
+    selection.clear();
+
+    // Negative waste when the long term fee is greater than the current fee and the selected value == target
+    const CAmount exact_target1{3 * COIN - 2 * fee};
+    const CAmount target_waste1{-2 * fee_diff}; // = (2 * fee) - (2 * (fee + fee_diff))
+    add_coin(1 * COIN, 1, selection, fee, fee + fee_diff);
+    add_coin(2 * COIN, 2, selection, fee, fee + fee_diff);
+    BOOST_CHECK_EQUAL(target_waste1, GetSelectionWaste(selection, /*change_cost=*/ 0, exact_target1));
+    selection.clear();
+
+    // Negative waste when the long term fee is greater than the current fee and change_cost < - (inputs * (fee - long_term_fee))
+    const CAmount large_fee_diff{90};
+    const CAmount target_waste2{-2 * large_fee_diff + change_cost}; // = (2 * fee) - (2 * (fee + large_fee_diff)) + change_cost
+    add_coin(1 * COIN, 1, selection, fee, fee + large_fee_diff);
+    add_coin(2 * COIN, 2, selection, fee, fee + large_fee_diff);
+    BOOST_CHECK_EQUAL(target_waste2, GetSelectionWaste(selection, change_cost, target));
 }
 
 BOOST_AUTO_TEST_CASE(effective_value_test)