all-packages.nix: Don't recurse into 'tests'

Doing so significantly slows down 'nix-env -qa' (4.30s vs 5.17s),
mostly because it seems to do a NixOS system evaluation.

This attribute should probably be removed entirely since tests should
be in release.nix, not pollute nix-env's package set.

pkgs/top-level/all-packages.nix

@@ -84,7 +84,7 @@ in
     by Hydra.
   '';
 
-  tests = recurseIntoAttrs (callPackages ../test {});
+  tests = callPackages ../test {};
 
   ### Nixpkgs maintainer tools