Compile testsuite with -threaded (haskell#264)

Fixes haskell#264.

happy.cabal

@@ -161,6 +161,7 @@ test-suite tests
   main-is: test.hs
   -- This line is important as it ensures that the local `exe:happy` component declared above is built before the test-suite component is invoked, as well as making sure that `happy` is made available on $PATH and `$happy_datadir` is set accordingly before invoking `test.hs`
   build-tools: happy
+  ghc-options: -threaded
 
   build-depends: base, process
   default-language: Haskell98

test.hs

@@ -4,6 +4,6 @@ import System.Process (system)
 import System.Exit (exitWith)
 
 main = do
-  let jFlag = "-j" ++ show numCapabilities
+  let jFlag = "-j" ++ show numCapabilities -- to run tests in parallel, run `cabal test --test-options="+RTS -N"`
   let cmd = ["make", jFlag, "-k", "-C", "tests", "clean", "all"]
   system (intercalate " " cmd) >>= exitWith