Test the extra packages here too

Probably dumb to have things split like this

nixos/lib/tests/test_pspackages.nix

+{ pspkgs }:
+{
+  test_tahoe_exists = {
+    expected = "1.14.0.dev";
+    expr = pspkgs.python2.pkgs.tahoe-lafs.version;
+  };
+  test_zkapauthorizer_exists = {
+    expected = "0.0";
+    expr = pspkgs.python2.pkgs.zkapauthorizer.version;
+  };
+  test_privatestorage_exists = {
+    expected = true;
+    expr = pspkgs ? privatestorage;
+  };
+}

nixos/unit-tests.nix

@@ -11,6 +11,9 @@ let
   [ (import ./lib/tests/test_ini.nix (pkgs.callPackage ./lib/ini.nix { }))
     (import ./lib/tests/test_openssl.nix pkgs.openssl)
     (import ./lib/tests/test_packages.nix { inherit pkgs; })
+    (import ./lib/tests/test_pspackages.nix {
+      pspkgs = pkgs.callPackage ./modules/pspkgs.nix { };
+    })
   ];
 
   # Count up the tests we're going to run.