wallet: make OutputGroup "positive_only" filter explicit

And not hide it inside the `OutputGroup::Insert` method.
This method does not return anything if insertion fails.

We can know before calling `Insert` whether the coin
will be accepted or not.

src/wallet/test/coinselector_tests.cpp

@@ -53,7 +53,7 @@ static void add_coin(const CAmount& nValue, int nInput, SelectionResult& result)
     tx.nLockTime = nextLockTime++;        // so all transactions get different hashes
     COutput output(COutPoint(tx.GetHash(), nInput), tx.vout.at(nInput), /*depth=*/ 1, /*input_bytes=*/ -1, /*spendable=*/ true, /*solvable=*/ true, /*safe=*/ true, /*time=*/ 0, /*from_me=*/ false, /*fees=*/ 0);
     OutputGroup group;
-    group.Insert(output, /*ancestors=*/ 0, /*descendants=*/ 0, /*positive_only=*/ true);
+    group.Insert(output, /*ancestors=*/ 0, /*descendants=*/ 0);
     result.AddInput(group);
 }
 
@@ -134,7 +134,7 @@ inline std::vector<OutputGroup>& GroupCoins(const std::vector<COutput>& availabl
     static_groups.clear();
     for (auto& coin : available_coins) {
         static_groups.emplace_back();
-        static_groups.back().Insert(coin, /*ancestors=*/ 0, /*descendants=*/ 0, /*positive_only=*/ false);
+        static_groups.back().Insert(coin, /*ancestors=*/ 0, /*descendants=*/ 0);
     }
     return static_groups;
 }

src/wallet/test/fuzz/coinselection.cpp

@@ -29,8 +29,10 @@ static void GroupCoins(FuzzedDataProvider& fuzzed_data_provider, const std::vect
     auto output_group = OutputGroup(coin_params);
     bool valid_outputgroup{false};
     for (auto& coin : coins) {
-        output_group.Insert(coin, /*ancestors=*/0, /*descendants=*/0, positive_only);
-        // If positive_only was specified, nothing may have been inserted, leading to an empty output group
+        if (!positive_only || (positive_only && coin.GetEffectiveValue() > 0)) {
+            output_group.Insert(coin, /*ancestors=*/0, /*descendants=*/0);
+        }
+        // If positive_only was specified, nothing was inserted, leading to an empty output group
         // that would be invalid for the BnB algorithm
         valid_outputgroup = !positive_only || output_group.GetSelectionAmount() > 0;
         if (valid_outputgroup && fuzzed_data_provider.ConsumeBool()) {