opt: Skip branches with worse weight

Once we exceed the weight of the current best selection, we can always
shift as adding more inputs can never yield a better solution.

Author: murchandamus

src/wallet/coinselection.cpp

@@ -413,6 +413,9 @@ util::Result<SelectionResult> CoinGrinder(std::vector<OutputGroup>& utxo_pool, c
             // max_weight exceeded: SHIFT
             max_tx_weight_exceeded = true;
             should_shift  = true;
+        } else if (curr_weight > best_selection_weight) {
+            // Worse weight than best solution. More UTXOs only increase weight: SHIFT
+            should_shift  = true;
         } else if (curr_amount >= total_target) {
             // Success, adding more weight cannot be better: SHIFT
             should_shift  = true;

src/wallet/test/coinselector_tests.cpp

@@ -1231,7 +1231,7 @@ BOOST_AUTO_TEST_CASE(coin_grinder_tests)
         add_coin(4 * COIN, 3, expected_result);
         BOOST_CHECK(EquivalentResult(expected_result, *res));
         // Demonstrate how following improvements reduce iteration count and catch any regressions in the future.
-        size_t expected_attempts = 2041;
+        size_t expected_attempts = 525;
         BOOST_CHECK_MESSAGE(res->GetSelectionsEvaluated() == expected_attempts, strprintf("Expected %i attempts, but got %i", expected_attempts, res->GetSelectionsEvaluated()));
     }