nixos/xss-lock: add module (#40619)

`xsslock` (which was originally packaged in 6cb1d1aaaf02a72329bedf9c6960e54fea6f5c6e)
is a simple screensaver which connects a given screen locker (e.g.
`i3lock`) with `logind`. Whenever `loginctl lock-sessions` is invoked
the locker will be used to lock the screen. This works with its power
management features (e.g. `lid switch`) as well, so the PC can be locked
automatically when the lid is closed.

The module can be used like this:

```
{
  services.xserver.enable = true;

  programs.xss-lock.enable = true;
  programs.xss-lock.lockerCommand = "i3lock";
}
```

1 files changed, 25 insertions, 0 deletions

diff --git a/nixos/tests/xss-lock.nix b/nixos/tests/xss-lock.nix
new file mode 100644
index 00000000000..3e3864cab77
--- /dev/null
+++ b/nixos/tests/xss-lock.nix
@@ -0,0 +1,25 @@
+import ./make-test.nix ({ pkgs, lib, ... }:
+
+with lib;
+
+{
+  name = "xss-lock";
+  meta.maintainers = with pkgs.stdenv.lib.maintainers; [ ma27 ];
+
+  machine = {
+    imports = [ ./common/x11.nix ./common/user-account.nix ];
+    programs.xss-lock.enable = true;
+    programs.xss-lock.lockerCommand = "${pkgs.xlockmore}/bin/xlock";
+    services.xserver.displayManager.auto.user = "alice";
+  };
+
+  testScript = ''
+    $machine->start;
+    $machine->waitForX;
+    $machine->waitForUnit("xss-lock.service", "alice");
+
+    $machine->fail("pgrep xlock");
+    $machine->succeed("su -l alice -c 'xset dpms force standby'");
+    $machine->succeed("pgrep xlock");
+  '';
+})