Only check examples with a corresponding .out file

This allows files to exists in the examples directory that don't have
a ".out" file. These are now ignored in the test suite. Such files
could be documentation or ephemeral tests.

ma.cabal

@@ -157,3 +157,4 @@ test-suite examples
                      , hspec
                      , directory
                      , text
+                     , extra

tests/Examples.hs

@@ -5,6 +5,7 @@ module Main (main) where
 import           Control.Monad.ST
 import           Data.List
 
+import           Data.List.Extra (dropSuffix)
 import qualified Data.Text as T
 import           System.Directory
 import           Test.Hspec
@@ -16,7 +17,7 @@ import           MA.PartitionPrinter
 main :: IO ()
 main = do
   let exampleDir = "examples/"
-  files <- filter (not . isOutput) <$> listDirectory exampleDir
+  files <- filter isOutput <$> listDirectory exampleDir
   hspec $ testFiles (map (exampleDir ++) files)
 
 outSuffix :: String
@@ -29,7 +30,9 @@ testFiles :: [FilePath] -> Spec
 testFiles = mapM_ testFile
 
 testFile :: FilePath -> Spec
-testFile file = specify file $ do
+testFile outfile =
+  let file = dropSuffix outSuffix outfile
+   in specify file $ do
         output <- readFile (file ++ outSuffix)
         process file `shouldReturn` output