Fanout Mutation: Set utxoToDecommit in the fanout snapshot

Exercise the path where the fanout snapshot is created with something to
decommit and then alter the outputs to add one more so the hash doesn't
match with the fanout redeemer.

Author: v0d1ch

hydra-node/test/Hydra/Chain/Direct/Contract/FanOut.hs

@@ -10,6 +10,7 @@ import Cardano.Api.UTxO as UTxO
 import Hydra.Chain.Direct.Contract.Mutation (Mutation (..), SomeMutation (..), changeMintedTokens)
 import Hydra.Chain.Direct.Fixture (slotLength, systemStart, testNetworkId, testPolicyId, testSeedInput)
 import Hydra.Chain.Direct.ScriptRegistry (genScriptRegistry, registryUTxO)
+import Hydra.Chain.Direct.State (splitUTxO)
 import Hydra.Chain.Direct.Tx (fanoutTx, mkHeadOutput)
 import Hydra.Contract.Error (toErrorCode)
 import Hydra.Contract.HeadError (HeadError (..))
@@ -42,9 +43,8 @@ healthyFanoutTx =
   tx =
     fanoutTx
       scriptRegistry
-      healthyFanoutUTxO
-      -- TODO: add something to decommit here
-      Nothing
+      (fst healthyFanoutSnapshotUTxO)
+      (Just $ snd healthyFanoutSnapshotUTxO)
       (headInput, headOutput)
       healthySlotNo
       headTokenScript
@@ -79,12 +79,15 @@ healthyContestationDeadline :: UTCTime
 healthyContestationDeadline =
   slotNoToUTCTime systemStart slotLength $ healthySlotNo - 1
 
+healthyFanoutSnapshotUTxO :: (UTxO, UTxO)
+healthyFanoutSnapshotUTxO = generateWith (splitUTxO healthyFanoutUTxO) 42
+
 healthyFanoutDatum :: Head.State
 healthyFanoutDatum =
   Head.Closed
     { snapshotNumber = 1
-    , utxoHash = toBuiltin $ hashUTxO @Tx healthyFanoutUTxO
-    , utxoToDecommitHash = toBuiltin $ hashUTxO @Tx mempty
+    , utxoHash = toBuiltin $ hashUTxO @Tx (fst healthyFanoutSnapshotUTxO)
+    , utxoToDecommitHash = toBuiltin $ hashUTxO @Tx (snd healthyFanoutSnapshotUTxO)
     , parties =
         partyToChain <$> healthyParties
     , contestationDeadline = posixFromUTCTime healthyContestationDeadline
@@ -129,6 +132,8 @@ genFanoutMutation (tx, _utxo) =
     , SomeMutation (pure $ toErrorCode BurntTokenNumberMismatch) MutateThreadTokenQuantity <$> do
         (token, _) <- elements burntTokens
         changeMintedTokens tx (valueFromList [(token, 1)])
+    , SomeMutation (pure $ toErrorCode FannedOutUtxoHashNotEqualToClosedUtxoHash) MutateAddUnexpectedOutput . PrependOutput <$> do
+        arbitrary >>= genOutput
     ]
  where
   burntTokens =