lib.nixos.runTest: Return the test itself

(NB: the lib in the flake)

Correcting a mistake I made when removing the test matrix code
from https://github.com/NixOS/nixpkgs/pull/191540

nixos/lib/testing/call-test.nix

@@ -10,7 +10,7 @@ in
     };
     result = mkOption {
       internal = true;
-      default = config;
+      default = config.test;
     };
   };
 }