lib/tests/test-with-nix.nix: init entrypoint for NixOS/nix CI

[roberth]

Description of changes

Provide a file that the nix CI can import. This helps ascertain the release worthiness of Nix itself during its development.
It previously used the following, but that broke somewhat recently.

              import (nixpkgs + "/lib/tests/release.nix")
                { inherit pkgs;
                  nixVersions = [ nix ];
                }

This is intended to be used on the Nix side in

• ~Add nixos-unstable based outputs, bdwgc 8.2.4~, Nixpkgs lib tests, cleanup nix#9900

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandboxing enabled in nix.conf? (See Nix manual)
  • sandbox = relaxed
  • sandbox = true
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 24.05 Release Notes (or backporting 23.05 and 23.11 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

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[infinisil]

LGTM!