Documentation: NixOS VM tests on macOS

[fricklerhandwerk]

Problem

#108984 added the capability running NixOS VM tests from macOS, but it seems nowhere documented. There is the virtualisation.host.pkgs option, but that seems not to be related to the VM test implementation. How does one set up NixOS VM tests on macOS so we can advertise and teach it on nix.dev?

Proposal

Add an example and some prose to the options documentation of the test driver.

Checklist

• checked latest Nixpkgs manual (source) and latest NixOS manual (source)
• checked open documentation issues for possible duplicates
• tried to find open documentation pull requests for possible solutions, but search results are garbage

[nixos-discourse]

This issue has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/run-nixos-interactive-tests-on-aarch64-darwin/34534/1