nixos/lib/testing: avoid generating darwin VM tests

We're getting 2x5 darwin VM jobs that aren't schedulable
on our current Hydra.nixos.org, which makes them hang around
and delay advancing of all `nixpkgs-*` channels.
To me that's quite an annoying effect, as it can be like an extra day
of additional delay without any benefit that I can really perceive.
(unless someone like me keeps manually cancelling the jobs all the time)

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