message: 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.
diff: @@ -136,6 +136,13 @@ 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)
  {
  