Merge bitcoin/bitcoin#24530: wallet: assert BnB's internally calculated waste is the same as GetSelectionWaste

ec7d73628a [wallet] assert BnB internally calculated waste is the same as GetSelectionWaste() (glozow)

Pull request description:

  #22009 introduced a `GetSelectionWaste()` function to determine how much "waste" a coin selection solution has, and is a mirror of the waste calculated inside of `SelectCoinsBnB()`. It would be bad for these two waste metrics to deviate, since it could negatively affect how often we select the BnB solution. Add an assertion to help tests catch a potential accidental change.

ACKs for top commit:
  achow101:
    ACK ec7d73628a
  Xekyo:
    ACK ec7d73628a

Tree-SHA512: 3ab7ad45ae92e7ce3f21824fb975105b6be8382edf47c252df5d13d973a3abdcb675132d223b42fcbb669cca879672c904b8a58d0676e12bf381a9219f4db37c

src/wallet/coinselection.cpp

@@ -163,6 +163,8 @@ std::optional<SelectionResult> SelectCoinsBnB(std::vector<OutputGroup>& utxo_poo
             result.AddInput(utxo_pool.at(i));
         }
     }
+    result.ComputeAndSetWaste(CAmount{0});
+    assert(best_waste == result.GetWaste());
 
     return result;
 }

src/wallet/spend.cpp

@@ -379,7 +379,6 @@ std::optional<SelectionResult> AttemptSelection(const CWallet& wallet, const CAm
     // Note that unlike KnapsackSolver, we do not include the fee for creating a change output as BnB will not create a change output.
     std::vector<OutputGroup> positive_groups = GroupOutputs(wallet, coins, coin_selection_params, eligibility_filter, true /* positive_only */);
     if (auto bnb_result{SelectCoinsBnB(positive_groups, nTargetValue, coin_selection_params.m_cost_of_change)}) {
-        bnb_result->ComputeAndSetWaste(CAmount(0));
         results.push_back(*bnb_result);
     }