macOS support for NixOS tests (#282401)

Closes #193336
Closes #261694
Related to #108984

The goal here was to get the following flake to build and run on
`aarch64-darwin`:

```nix
{ inputs.nixpkgs.url = <this branch>;

  outputs = { nixpkgs, ... }: {
    checks.aarch64-darwin.default =
      nixpkgs.legacyPackages.aarch64-darwin.nixosTest {
        name = "test";

        nodes.machine = { };

        testScript = "";
      };
  };
}
```

… and after this change it does.  There's no longer a need for the
user to set `nodes.*.nixpkgs.pkgs` or
`nodes.*.virtualisation.host.pkgs` as the correct values are inferred
from the host system.

nixos/lib/testing/nodes.nix

@@ -14,6 +14,25 @@ let
     types
     ;
 
+  inherit (hostPkgs) hostPlatform;
+
+  guestSystem =
+    if hostPlatform.isLinux
+    then hostPlatform.system
+    else
+      let
+        hostToGuest = {
+          "x86_64-darwin" = "x86_64-linux";
+          "aarch64-darwin" = "aarch64-linux";
+        };
+
+        supportedHosts = lib.concatStringsSep ", " (lib.attrNames hostToGuest);
+
+        message =
+          "NixOS Test: don't know which VM guest system to pair with VM host system: ${hostPlatform.system}. Perhaps you intended to run the tests on a Linux host, or one of the following systems that may run NixOS tests: ${supportedHosts}";
+      in
+        hostToGuest.${hostPlatform.system} or (throw message);
+
   baseOS =
     import ../eval-config.nix {
       inherit lib;
@@ -27,13 +46,14 @@ let
           ({ config, ... }:
             {
               virtualisation.qemu.package = testModuleArgs.config.qemu.package;
+              virtualisation.host.pkgs = hostPkgs;
             })
           ({ options, ... }: {
             key = "nodes.nix-pkgs";
             config = optionalAttrs (!config.node.pkgsReadOnly) (
               mkIf (!options.nixpkgs.pkgs.isDefined) {
                 # TODO: switch to nixpkgs.hostPlatform and make sure containers-imperative test still evaluates.
-                nixpkgs.system = hostPkgs.stdenv.hostPlatform.system;
+                nixpkgs.system = guestSystem;
               }
             );
           })

nixos/lib/testing/pkgs.nix

@@ -2,7 +2,11 @@
 {
   config = {
     # default pkgs for use in VMs
-    _module.args.pkgs = hostPkgs;
+    _module.args.pkgs =
+      # TODO: deprecate it everywhere; not just on darwin. Throw on darwin?
+      lib.warnIf hostPkgs.stdenv.hostPlatform.isDarwin
+        "Do not use the `pkgs` module argument in tests you want to run on darwin. It is ambiguous, and many tests are broken because of it. If you need to use a package on the VM host, use `hostPkgs`. Otherwise, use `config.node.pkgs`, or `config.nodes.<name>.nixpkgs.pkgs`."
+        hostPkgs;
 
     defaults = {
       # TODO: a module to set a shared pkgs, if options.nixpkgs.* is untouched by user (highestPrio) */

nixos/lib/testing/run.nix

@@ -41,7 +41,9 @@ in
     rawTestDerivation = hostPkgs.stdenv.mkDerivation {
       name = "vm-test-run-${config.name}";
 
-      requiredSystemFeatures = [ "kvm" "nixos-test" ];
+      requiredSystemFeatures = [ "nixos-test" ]
+        ++ lib.optionals hostPkgs.stdenv.hostPlatform.isLinux [ "kvm" ]
+        ++ lib.optionals hostPkgs.stdenv.hostPlatform.isDarwin [ "apple-virt" ];
 
       buildCommand = ''
         mkdir -p $out