nixos/testing: Add onVMHost attribute to all tests

[roberth]

The interaction between pkg.override* and alternate VM hosts does not work.
Concretely, if I'm using Darwin, and I'm developing a patch to a linux package, I'd want to do (pkg.overrideAttrs someFunction).tests, but this does not work, because that will (have to) use the Linux version of the package, whereas we've added our patch (in someFunction) to a Darwin instantiation of the package.
The test framework can compensate for this.

Instead, with this PR, you could override the Linux package, and then build something like (pkg.overrideAttrs someFunction).tests.nixos.onVMHost.aarch64-darwin (where pkg is for Linux).

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/)
• Nixpkgs 25.11 Release Notes (or backporting 25.05 Nixpkgs Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
• NixOS 25.11 Release Notes (or backporting 25.05 NixOS Release notes)
  • (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, pkgs/README.md, maintainers/README.md and other contributing documentation in corresponding paths.

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[wolfgangwalther]

I'm thinking about how to use this while developing for linux from darwin. Am I going to think about anything "cross" or am I thinking "native linux" instead? What I'm after is: Could onVMHost be misleading, depending on context? I understand "Host" to be the host-vs-guest for the VM - but there is also a Build/Host/Target "Host".

and then build something like (pkg.overrideAttrs someFunction).tests.nixos.onVMHost.aarch64-darwin (where pkg is for Linux).

Also I wonder how this "is for linux" comes into play. How would this look like in practice, aka a full call? Just to get an idea of how confusing that might be.

[roberth]

Also I wonder how this "is for linux" comes into play.

Complete example:

nix build .#legacyPackages.aarch64-linux.podman.tests.oci-containers-podman.onVMHost.aarch64-darwin

Am I going to think about anything "cross"

No. "VM Host" is one term here, and it's the same "host" as in hostPkgs. Not great, but no idea what else to call it, and changing hostPkgs would be a project, but I'm open to suggestions.