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

These two implementations of waste calculation should never deviate.
Still keep the SelectCoinsBnB internal calculation because incremental
calculate-as-you-go is much more performant than calling
GetSelectionWaste() over and over again.

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;
 }