Testing functions in Haskell that do IO

Working through Real World Haskell right now. Here's a solution to a very early exercise in the book:

-- | 4) Counts the number of characters in a file
numCharactersInFile :: FilePath -> IO Int
numCharactersInFile fileName = do
    contents <- readFile fileName
    return (length contents)

My question is: How would you test this function? Is there a way to make a "mock" input instead of actually needing to interact with the file system to test it out? Haskell places such an emphasis on pure functions that I have to imagine that this is easy to do.