haskellPackages.cryptonite: re-enable tests.

This has been fixed since cryptonite-0.7:

haskell-crypto/cryptonite#28

cryptonite is also no longer listed in expected-test-failures in
Stackage.

pkgs/development/haskell-modules/configuration-common.nix

@@ -676,9 +676,6 @@ self: super: {
   # https://github.com/goldfirere/singletons/issues/122
   singletons = dontCheck super.singletons;
 
-  # https://github.com/fpco/stackage/issues/838
-  cryptonite = dontCheck super.cryptonite;
-
   # We cannot build this package w/o the C library from <http://www.phash.org/>.
   phash = markBroken super.phash;