flake: Attempt to run doctests

Reviewed-by: Alexander Kobjolke <me@failco.de>

test/doctest/Doctest.hs

@@ -0,0 +1,10 @@
+-- | Rn the doctest executable
+module Main where
+
+import System.Process (callProcess)
+
+doctest :: [String] -> IO ()
+doctest = callProcess "doctest"
+
+main :: IO ()
+main = doctest ["--fast", "src"]