Merge bitcoin/bitcoin#22938: test: Add remaining scenarios of 0 waste, in wallet waste_test

efcaefc test: Add remaining scenarios of 0 waste (rajarshimaitra)

Pull request description:

  As per the [review club](https://bitcoincore.reviews/22009) discussion on #22009 , it was observed that there were other two fee scenarios in which selection waste could be zero.

  These are:
   - (LTF - Fee) == Change Cost
   - (LTF - Fee) == Excess

  Even though these are obvious by the definition of waste metric, adding tests for them can be helpful in explaining its behavior
  to new readers of the code base, along with pinning the behavior for future.

  This PR adds those two cases to waste calculation unit test.

  Also let me know if I am missing more scenarios.

ACKs for top commit:
  jonatack:
    Tested re-ACK efcaefc
  achow101:
    ACK efcaefc
  meshcollider:
    ACK efcaefc

Tree-SHA512: 13fe3e2c0ea7bb58d34e16c32908b84705130dec16382ff941e5e60ca5b379f9c5811b33f36c4c72d7a98cfbb6af2f196d0a69e96989afa4b9e49893eaadd7cb

Author: meshcollider

src/wallet/test/coinselector_tests.cpp

@@ -724,12 +724,25 @@ 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()