modules/nixpkgs: construct an instance of nixpkgs.source

tests/nixpkgs-mock.nix

@@ -0,0 +1,15 @@
+# This mock nixpkgs can be used as `nixpkgs.source` in nixpkgs-module-test
+# if we want/need to avoid importing & instantiating a real nixpkgs
+{
+  config ? { },
+  ...
+}:
+let
+  pkgs = {
+    _type = "pkgs";
+    __splicedPackages = pkgs;
+    inherit config pkgs;
+    mock = true;
+  };
+in
+pkgs

tests/nixpkgs-module.nix

@@ -36,7 +36,51 @@ let
 in
 linkFarmFromDrvs "nixpkgs-module-test" [
 
-  # TODO: expect not setting `nixpkgs.pkgs` to throw
+  # Test that pkgs-config is affected by `nixpkgs.config`
+  (testModule "nixpkgs-config" (
+    { pkgs, ... }:
+    {
+      nixpkgs.config = {
+        permittedInsecurePackages = [
+          "foobar123"
+        ];
+      };
+
+      nixpkgs.hostPlatform = {
+        inherit (stdenv.hostPlatform) system;
+      };
+
+      assertions = [
+        {
+          assertion = pkgs.config.permittedInsecurePackages == [ "foobar123" ];
+          message = ''
+            Expected `pkgs.config.permittedInsecurePackages` to match [ "foobar123" ], but found:
+            ${lib.generators.toPretty { } pkgs.config.permittedInsecurePackages}'';
+        }
+      ];
+    }
+  ))
+
+  # Test that a nixpkgs revision can be specified using `nixpkgs.source`
+  (testModule "nixpkgs-source" (
+    { pkgs, ... }:
+    {
+      nixpkgs.source = ./nixpkgs-mock.nix;
+
+      nixpkgs.hostPlatform = {
+        inherit (stdenv.hostPlatform) system;
+      };
+
+      assertions = [
+        {
+          assertion = pkgs.mock or false;
+          message = "Expected `pkgs.mock` to be true, but ${
+            if pkgs ? mock then "found " + lib.generators.toPretty { } pkgs.mock else "isn't present"
+          }";
+        }
+      ];
+    }
+  ))
 
   (testModule "nixpkgs-overlays" (
     { pkgs, ... }: