nixos/testing: Add test attribute to please discoverTests (#423272)

nixos/lib/testing/run.nix

@@ -106,6 +106,11 @@ in
     # useful for inspection (debugging / exploration)
     passthru.config = config;
 
+    /**
+      For discoverTests only. Deprecated. Will be removed when discoverTests can be removed from NixOS all-tests.nix.
+    */
+    passthru.test = config.test;
+
     # Docs: nixos/doc/manual/development/writing-nixos-tests.section.md
     /**
       See https://nixos.org/manual/nixos/unstable#sec-override-nixos-test

nixos/tests/all-tests.nix

@@ -18,10 +18,12 @@
 with pkgs.lib;
 
 let
+  # TODO: remove when handleTest is gone (make sure nixosTests and nixos/release.nix#tests are unaffected)
+  # TODO: when removing, also deprecate `test` attribute in ../lib/testing/run.nix
   discoverTests =
     val:
     if isAttrs val then
-      if hasAttr "test" val then
+      if (val ? test) then
         callTest val
       else
         mapAttrs (n: s: if n == "passthru" then s else discoverTests s) val