libidn: re-enable tests

Tests where disabled for a long time, but they seem to pass now.

Succesfully built libidn and pkgsMusl.libidn on linux-x86_64.

pkgs/development/libraries/libidn/default.nix

@@ -11,15 +11,10 @@ stdenv.mkDerivation rec {
 
   outputs = [ "bin" "dev" "out" "info" "devdoc" ];
 
-  # broken with gcc-7
-  #doCheck = !stdenv.isDarwin && !stdenv.hostPlatform.isMusl;
-
   hardeningDisable = [ "format" ];
 
   buildInputs = lib.optional stdenv.isDarwin libiconv;
 
-  doCheck = false; # fails
-
   meta = {
     homepage = "https://www.gnu.org/software/libidn/";
     description = "Library for internationalized domain names";