bnb: exit selection when best_waste is 0

If we find a solution which has no waste, just use that. This solution
is what we would consider to be optimal, and other solutions we find
would have to also have 0 waste, so they are equivalent to the first
one with 0 waste. Thus we can optimize by just choosing the first one
with 0 waste.

src/wallet/coinselection.cpp

@@ -106,6 +106,9 @@ bool SelectCoinsBnB(std::vector<OutputGroup>& utxo_pool, const CAmount& target_v
                 best_selection = curr_selection;
                 best_selection.resize(utxo_pool.size());
                 best_waste = curr_waste;
+                if (best_waste == 0) {
+                    break;
+                }
             }
             curr_waste -= (curr_value - actual_target); // Remove the excess value as we will be selecting different coins now
             backtrack = true;

src/wallet/test/coinselector_tests.cpp

@@ -176,8 +176,8 @@ BOOST_AUTO_TEST_CASE(bnb_search_test)
     selection.clear();
 
     // Select 5 Cent
-    add_coin(3 * CENT, 3, actual_selection);
-    add_coin(2 * CENT, 2, actual_selection);
+    add_coin(4 * CENT, 4, actual_selection);
+    add_coin(1 * CENT, 1, actual_selection);
     BOOST_CHECK(SelectCoinsBnB(GroupCoins(utxo_pool), 5 * CENT, 0.5 * CENT, selection, value_ret, not_input_fees));
     BOOST_CHECK(equal_sets(selection, actual_selection));
     BOOST_CHECK_EQUAL(value_ret, 5 * CENT);
@@ -204,9 +204,8 @@ BOOST_AUTO_TEST_CASE(bnb_search_test)
 
     // Select 10 Cent
     add_coin(5 * CENT, 5, utxo_pool);
+    add_coin(5 * CENT, 5, actual_selection);
     add_coin(4 * CENT, 4, actual_selection);
-    add_coin(3 * CENT, 3, actual_selection);
-    add_coin(2 * CENT, 2, actual_selection);
     add_coin(1 * CENT, 1, actual_selection);
     BOOST_CHECK(SelectCoinsBnB(GroupCoins(utxo_pool), 10 * CENT, 0.5 * CENT, selection, value_ret, not_input_fees));
     BOOST_CHECK(equal_sets(selection, actual_selection));