tests: Add KnapsackGroupOutputs helper function

In order to change the KnapsackSolver tests to call KnapsackSolver, we
need KnapsackGroupOutputs to create the OutputGroups filtered with the
filter criteria.

src/wallet/test/coinselector_tests.cpp

@@ -137,6 +137,13 @@ inline std::vector<OutputGroup>& GroupCoins(const std::vector<COutput>& coins)
     return static_groups;
 }
 
+inline std::vector<OutputGroup>& KnapsackGroupOutputs(const CoinEligibilityFilter& filter)
+{
+    static std::vector<OutputGroup> static_groups;
+    static_groups = testWallet.GroupOutputs(vCoins, coin_selection_params, filter, /* positive_only */false);
+    return static_groups;
+}
+
 // Branch and bound coin selection tests
 BOOST_AUTO_TEST_CASE(bnb_search_test)
 {