nixos/tests/openldap: fix evaluation when auto-called

[sternenseemann]

The NixOS manual documents that you can invoke every tests using

nix-build path/to/nixos/tests/test.nix

which was not the case for openldap since it is not autocallable, but
requires pkgs and system as arguments. Usually, make-test-pythons.nix
takes care of this if it is imported at the top-level, but since
openldap.nix contains multiple tests, this was not the case.

This is however easily fixed by:

• Adding default values for the pkgs and system arguments based on the
  definition in make-test-python.nix
• Passing pkgs and system explicitly to make-test-python.nix to ensure
  the pkgs and system values passed from all-tests.nix are used.

Motivation for this change

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.