Add Literate Haskell syntax test file

tests/syntax-tests/source/Literate Haskell/Main.lhs

@@ -0,0 +1,47 @@
+\documentclass{article}
+
+\begin{document}
+
+\section*{Introduction}
+
+Text outside code environments should follow TeX/LaTeX highlighting.
+
+The code environment delimiters themselves should be highlighted.
+
+Text inside code environments should follow regular Haskell highlighting.
+
+\begin{code}
+import           Data.List
+import           System.Environment
+import           Text.Printf
+
+twoSumN :: Int -> [Int] -> [Int]
+twoSumN _ [] = []
+twoSumN n (x : xs) | (n - x) `elem` xs = [x, n - x]
+                   | otherwise         = twoSumN n xs
+
+threeSumN :: Int -> [Int] -> [Int]
+threeSumN _ [] = []
+threeSumN n (x : xs) | null partial = threeSumN n xs
+                     | otherwise    = x : partial
+  where partial = twoSumN (n - x) xs
+\end{code}
+
+Text in-between code environments.
+% LaTeX comment.
+
+\begin{code}
+output :: String -> IO ()
+output path = do
+  input <- sort . map read . filter (not . null) . lines <$> readFile path
+  printf "File: %s\n" path
+  printf "  Part 1: %d\n" . product . twoSumN 2020 $ input
+  printf "  Part 2: %d\n" . product . threeSumN 2020 $ input
+
+-- Haskell comment inside code environment.
+
+main :: IO ()
+main = getArgs >>= mapM_ output
+\end{code}
+
+\end{document}