message: test: Add remaining scenarios of 0 waste
  There are two more cases where waste can be 0, when:
  - (Fee - LTF) == -Change Cost
  - (Fee - LTF) == -Excess
  Adding these two conditions explicitly in the unit test will help
  pin the behavior, also demonstrate waste calculation scenarios to new
  readers.
diff: @@ -717,12 +717,25 @@ BOOST_AUTO_TEST_CASE(waste_test)
  BOOST_CHECK_LT(waste_nochange2, waste_nochange1);
  selection.clear();
  - // 0 Waste only when fee == long term fee, no change, and no excess
  + // No Waste when fee == long_term_fee, no change, and no excess
  add_coin(1 * COIN, 1, selection, fee, fee);
  add_coin(2 * COIN, 2, selection, fee, fee);
  - const CAmount exact_target = in_amt - 2 * fee;
  - BOOST_CHECK_EQUAL(0, GetSelectionWaste(selection, 0, exact_target));
  + const CAmount exact_target{in_amt - fee * 2};
  + BOOST_CHECK_EQUAL(0, GetSelectionWaste(selection, /* change_cost */ 0, exact_target));
  + selection.clear();
  + // No Waste when (fee - long_term_fee) == (-cost_of_change), and no excess
  + const CAmount new_change_cost{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, new_change_cost, target));
  + selection.clear();
  +
  + // No Waste when (fee - long_term_fee) == (-excess), no change cost
  + 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_AUTO_TEST_SUITE_END()
  