Use Niv to pin nixpkgs releases

Before using Niv, we were following channels meaning we can not
reproduce CI jobs easily.

In this change, we use Niv to pin these dependencies. We are also
addding a tests/default.nix to be able to run these tests locally.

For instance, to run the test extern.nix on the nixpkgs-19.09 release:

    nix-build tests/default.nix -A extern.nixpkgs_19_09

Fixes #178

tests/clamav.nix

@@ -14,7 +14,9 @@
 #  You should have received a copy of the GNU General Public License
 #  along with this program. If not, see <http://www.gnu.org/licenses/>
 
-import <nixpkgs/nixos/tests/make-test.nix> {
+{ pkgs ? import <nixpkgs> {}}:
+
+import (pkgs.path + "/nixos/tests/make-test.nix") {
 
   nodes = {
     server = { config, pkgs, lib, ... }: