Fix MockChain submission during multiple rollbacks

There could be situations with race conditions when multiple
RollbackAndForward actions were generated by the model.

By using the correct "view" onto the chain, the mock chain can evaluate
transactions more consistently.

hydra-node/test/Hydra/Model/MockChain.hs

@@ -143,7 +143,12 @@ mockChainAndNetwork tr seedKeys commits = do
     -- Validate transactions on submission and queue them for inclusion if valid.
     let submitTx tx =
           atomically $ do
-            (_, _, _, utxo) <- readTVar chain
+            -- NOTE: Determine the current "view" on the chain (important while
+            -- rolled back, before new roll forwards were issued)
+            (_, position, blocks, globalUTxO) <- readTVar chain
+            let utxo = case Seq.lookup (fromIntegral position) blocks of
+                  Nothing -> globalUTxO
+                  Just (_, _, blockUTxO) -> blockUTxO
             -- TODO: dry with block tx validation
             case evaluateTx tx utxo of
               Left err ->