calamares: add .tests

This way r-ryantm will warn in their PRs, also CI I think.

pkgs/tools/misc/calamares/default.nix

@@ -3,6 +3,8 @@
 , qtbase, qtquickcontrols, qtsvg, qttools, qtwebengine, util-linux, tzdata
 , ckbcomp, xkeyboard_config, mkDerivation
 , nixos-extensions ? false
+# passthru.tests
+, calamares-nixos
 }:
 
 mkDerivation rec {
@@ -79,6 +81,10 @@ mkDerivation rec {
         -i CMakeLists.txt
   '';
 
+  passthru.tests = {
+    inherit calamares-nixos;
+  };
+
   meta = with lib; {
     description = "Distribution-independent installer framework";
     homepage = "https://calamares.io/";