Could use random test cases:

import Data.Char
import Test.QuickCheck

-- ...

  it "should work for random tests" $ property $
    forAll (listOf $ arbitrary `suchThat` isAscii) $ \s ->
      shortcut s `shouldBe` solution s

where solution = -- ...

Be more specific when you say "use fromIntegral"

Perhaps be a bit less obvious where you copied from next time: https://wiki.haskell.org/Foldl_as_foldr.
I mean, at least change the argument names.