now we can easily parameterize pkgs, too

default.nix

@@ -6,9 +6,9 @@
 
   # Run some system integration tests in VMs covering some of the software
   # we're integrating (ie, application functionality).
-  system-tests = import ./nixos/system-tests.nix;
+  system-tests = pkgs.callPackage ./nixos/system-tests.nix { };
 
   # Run some unit tests of the Nix that ties all of these things together (ie,
   # PrivateStorageio-internal library functionality).
-  unit-tests = import ./nixos/unit-tests.nix;
+  unit-tests = pkgs.callPackage ./nixos/unit-tests.nix { };
 }

nixos/system-tests.nix

 # The overall system test suite for PrivateStorageio NixOS configuration.
-let
-  pkgs = import ../nixpkgs-2105.nix { };
-in {
+{ pkgs }:
+{
   private-storage = pkgs.nixosTest ./tests/private-storage.nix;
   spending = pkgs.nixosTest ./tests/spending.nix;
   tahoe = pkgs.nixosTest ./tests/tahoe.nix;

nixos/unit-tests.nix

 # The overall unit test suite for PrivateStorageio NixOS configuration.
+{ pkgs }:
 let
-  pkgs = import <nixpkgs> { };
-
   # Total the numbers in a list.
   sum = builtins.foldl' (a: b: a + b) 0;