Add missing HasCallStack

Even though functions that use this one all have HasCallStack, the fact
that this function itself doesn't means that all errors are reported as
from this line anyway:

    Failures:

      ./Yesod/Test.hs:1571:28:
      1) ...

This should correct that.

yesod-test/Yesod/Test.hs

@@ -1567,7 +1567,7 @@ parseSetCookies :: [H.Header] -> [Cookie.SetCookie]
 parseSetCookies headers = map (Cookie.parseSetCookie . snd) $ DL.filter (("Set-Cookie"==) . fst) $ headers
 
 -- Yes, just a shortcut
-failure :: (MonadIO a) => T.Text -> a b
+failure :: (HasCallStack, MonadIO a) => T.Text -> a b
 failure reason = (liftIO $ HUnit.assertFailure $ T.unpack reason) >> error ""
 
 type TestApp site = (site, Middleware)