Is your feature request related to a problem? Please describe.

MacOS has two virtualization frameworks. By introducing these feature labels we allow for machines without such support to join a build farm.

Describe the solution you'd like

Add system-features = nixos-test to the default/example configuration and add "hvf" and "avf" dynamically based on whether these technologies are available, similar to how we do it for "kvm".

Describe alternatives you've considered

• Remove the requiredFeatures from Run nixosTests on darwin nixpkgs#193336 (comment)
  This will lead to problems when virtualized, non-standard or hardened darwin systems are part of a build cluster.

Additional context

Most MacOS systems support virtualization, but this is not a given.

AVF: https://developer.apple.com/documentation/bundleresources/entitlements/com_apple_security_virtualization

Priorities

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