Add assert for exit code

cabal-testsuite/Test/Cabal/Prelude.hs

@@ -602,6 +602,14 @@ getMarkedOutput out = unlines (go (lines out) False)
 assertFailure :: WithCallStack (String -> m ())
 assertFailure msg = withFrozenCallStack $ error msg
 
+assertExitCode :: MonadIO m => WithCallStack (ExitCode -> Result -> m ())
+assertExitCode code result =
+  when (code /= resultExitCode result) $
+    assertFailure $ "Expected exit code: "
+                 ++ show code
+                 ++ "\nActual: "
+                 ++ show (resultExitCode result)
+
 assertEqual :: (Eq a, Show a, MonadIO m) => WithCallStack (String -> a -> a -> m ())
 assertEqual s x y =
     withFrozenCallStack $