tests.testers.nixosTest-example: move from tests.nixos-functions.nixosTest-test

And improve the test a bit, to assert correct wiring of `pkgs`.

pkgs/build-support/testers/test/README.md

@@ -0,0 +1,8 @@
+# Tests _for the testers_
+
+    cd nixpkgs
+    nix-build -A tests.testers
+
+Tests generally derive their own correctness from simplicity, which in the
+case of testers (themselves functions) does not always work out.
+Hence the need for tests that test the testers.

pkgs/build-support/testers/test/default.nix

@@ -0,0 +1,27 @@
+{ testers, lib, pkgs, ... }:
+let
+  pkgs-with-overlay = pkgs.extend(final: prev: {
+    proof-of-overlay-hello = prev.hello;
+  });
+
+  dummyVersioning = {
+    revision = "test";
+    versionSuffix = "test";
+    label = "test";
+  };
+
+in
+lib.recurseIntoAttrs {
+  # Check that the wiring of nixosTest is correct.
+  # Correct operation of the NixOS test driver should be asserted elsewhere.
+  nixosTest-example = pkgs-with-overlay.testers.nixosTest ({ lib, pkgs, figlet, ... }: {
+    name = "nixosTest-test";
+    nodes.machine = { pkgs, ... }: {
+      system.nixos = dummyVersioning;
+      environment.systemPackages = [ pkgs.proof-of-overlay-hello figlet ];
+    };
+    testScript = ''
+      machine.succeed("hello | figlet >/dev/console")
+    '';
+  });
+}

pkgs/test/default.nix

@@ -67,6 +67,8 @@ with pkgs;
 
   writers = callPackage ../build-support/writers/test.nix {};
 
+  testers = callPackage ../build-support/testers/test/default.nix {};
+
   dhall = callPackage ./dhall { };
 
   makeWrapper = callPackage ./make-wrapper {};

pkgs/test/nixos-functions/default.nix

@@ -26,16 +26,5 @@ in lib.optionalAttrs stdenv.hostPlatform.isLinux (
       fileSystems."/".device = "/dev/null";
     }).toplevel;
 
-    nixosTest-test = pkgs.testers.nixosTest ({ lib, pkgs, figlet, ... }: {
-      name = "nixosTest-test";
-      nodes.machine = { pkgs, ... }: {
-        system.nixos = dummyVersioning;
-        environment.systemPackages = [ pkgs.hello figlet ];
-      };
-      testScript = ''
-        machine.succeed("hello | figlet >/dev/console")
-      '';
-    });
-
   }
 )