LedgerDB.StateMachine test: proper bookkeeping for rollbacks

Otherwise, the chain used on `DropAndRestore` gets malformed when there are
non-trivial rollbacks.

Author: amesgen

ouroboros-consensus/test/storage-test/Test/Ouroboros/Storage/LedgerDB/StateMachine.hs

@@ -553,7 +553,9 @@ instance RunModel Model (StateT Environment IO) where
         vr <- validateFork ldb rr (const $ pure ()) BlockCache.empty n (map getHeader blks)
         case vr of
           ValidateSuccessful forker -> do
-            atomically $ modifyTVar (dbChain chainDb) (reverse (map blockRealPoint blks) ++)
+            atomically $
+              modifyTVar (dbChain chainDb) $
+                (reverse (map blockRealPoint blks) ++) . drop (fromIntegral n)
             atomically (forkerCommit forker)
             forkerClose forker
           ValidateExceededRollBack{} -> error "Unexpected Rollback"