Introduce function reduceSelectionLimitBy.

This function reduces a selection limit by a given amount.

We use this function to replace the incorrect subtraction in
`adjustComputeSelectionLimit`.

Background:

The `subtract` function (from `Prelude`) is equivalent to `flip (-)`,
which means that `subtract a b` is equivalent to `b - a`, rather than
the other way round.

Thanks to @KtorZ for discovering this.

lib/core/src/Cardano/Wallet/Primitive/CoinSelection.hs

@@ -246,8 +246,8 @@ toBalanceConstraintsParams (constraints, params) =
             -- there is still space available.
             --
             adjustSelectionLimit :: SelectionLimit -> SelectionLimit
-            adjustSelectionLimit = fmap
-                (subtract (view #maximumCollateralInputCount constraints))
+            adjustSelectionLimit = flip Balance.reduceSelectionLimitBy
+                (view #maximumCollateralInputCount constraints)
 
     balanceParams = Balance.SelectionParams
         { assetsToBurn =

lib/core/src/Cardano/Wallet/Primitive/CoinSelection/Balance.hs

@@ -47,6 +47,7 @@ module Cardano.Wallet.Primitive.CoinSelection.Balance
     , SelectionLimitOf (..)
     , selectionLimitExceeded
     , SelectionLimitReachedError (..)
+    , reduceSelectionLimitBy
 
     -- * Querying selections
     , SelectionDelta (..)
@@ -442,6 +443,21 @@ selectionLimitExceeded s = \case
     NoLimit -> False
     MaximumInputLimit n -> UTxOSelection.selectedSize s > n
 
+-- | Reduces a selection limit by a given reduction amount.
+--
+-- If the given reduction amount is positive, then this function will reduce
+-- the selection limit by that amount.
+--
+-- If the given reduction amount is zero or negative, then this function will
+-- return the original limit unchanged.
+--
+reduceSelectionLimitBy :: SelectionLimit -> Int -> SelectionLimit
+reduceSelectionLimitBy limit reduction
+    | reduction <= 0 =
+        limit
+    | otherwise =
+        subtract reduction <$> limit
+
 type SelectionResult = SelectionResultOf [TxOut]
 
 -- | The result of performing a successful selection.