return an exit code from TAP.run

the value 70 is EX_SOFTWARE from bsd's sysexits.h. i was looking for an
excuse to use 69 but alas it was not to be

tests/TAP.idr

@@ -6,6 +6,7 @@ import Data.String
 import Data.List.Elem
 import Control.Monad.Reader
 import Control.Monad.State
+import System
 
 public export Info : Type
 Info = List (String, String)
@@ -144,7 +145,9 @@ mutual
 
 
 export
-run : List Test -> IO ()
+run : List Test -> IO ExitCode
 run tests = do
   putStrLn "TAP version 14"
-  ignore $ runReaderT 0 $ runList tests
+  pure $ if !(runReaderT 0 $ runList tests)
+            then ExitSuccess
+            else ExitFailure 70