nixos/testing-python.nix: Add evalTest

This is a decomposition of the testing-python.nix and build-vms.nix
files into modules.

By refactoring the glue, we accomplish the following:

 - NixOS tests can now use `imports` and other module system features.
    - Network-wide test setup can now be reusable; example:
       - A setup with all VMs configured to use a DNS server
       - Split long, slow tests into multiple tests that import a
         common module that has most of the setup.
    - Type checking for the test arguments
    - (TBD) "generated" options reference docs
 - Aspects that had to be wired through all the glue are now in their
   own files.
    - Chief example: interactive.nix.
    - Also: network.nix

In rewriting this, I've generally stuck as close as possible to the
existing code; copying pieces of logic and rewiring them, without
changing the logic itself.

I've made two exceptions to this rule

 - Introduction of `extraDriverArgs` instead of hardcoded
   interactivity logic.

 - Incorporation of https://github.com/NixOS/nixpkgs/pull/144110
   in testScript.nix.

I might revert the latter and split it into a new commit.

nixos/lib/testing/nodes.nix

@@ -0,0 +1,91 @@
+testModuleArgs@{ config, lib, hostPkgs, nodes, ... }:
+
+let
+  inherit (lib) mkOption mkForce optional types mapAttrs mkDefault;
+
+  system = hostPkgs.stdenv.hostPlatform.system;
+
+  baseOS =
+    import ../eval-config.nix {
+      inherit system;
+      inherit (config.node) specialArgs;
+      modules = [ config.defaults ];
+      baseModules = (import ../../modules/module-list.nix) ++
+        [
+          ../../modules/virtualisation/qemu-vm.nix
+          ../../modules/testing/test-instrumentation.nix # !!! should only get added for automated test runs
+          { key = "no-manual"; documentation.nixos.enable = false; }
+          {
+            key = "no-revision";
+            # Make the revision metadata constant, in order to avoid needless retesting.
+            # The human version (e.g. 21.05-pre) is left as is, because it is useful
+            # for external modules that test with e.g. testers.nixosTest and rely on that
+            # version number.
+            config.system.nixos.revision = mkForce "constant-nixos-revision";
+          }
+          { key = "nodes"; _module.args.nodes = nodes; }
+
+          ({ config, ... }:
+            {
+              virtualisation.qemu.package = testModuleArgs.config.qemu.package;
+
+              # Ensure we do not use aliases. Ideally this is only set
+              # when the test framework is used by Nixpkgs NixOS tests.
+              nixpkgs.config.allowAliases = false;
+            })
+        ] ++ optional config.minimal ../../modules/testing/minimal-kernel.nix;
+    };
+
+
+in
+
+{
+
+  options = {
+    node.type = mkOption {
+      type = types.raw;
+      default = baseOS.type;
+      internal = true;
+    };
+
+    nodes = mkOption {
+      type = types.lazyAttrsOf config.node.type;
+    };
+
+    defaults = mkOption {
+      description = ''
+        NixOS configuration that is applied to all {option}`nodes`.
+      '';
+      type = types.deferredModule;
+      default = { };
+    };
+
+    node.specialArgs = mkOption {
+      type = types.lazyAttrsOf types.raw;
+      default = { };
+    };
+
+    minimal = mkOption {
+      type = types.bool;
+      default = false;
+    };
+
+    nodesCompat = mkOption {
+      internal = true;
+    };
+  };
+
+  config = {
+    _module.args.nodes = config.nodesCompat;
+    nodesCompat =
+      mapAttrs
+        (name: config: config // {
+          config = lib.warn
+            "Module argument `nodes.${name}.config` is deprecated. Use `nodes.${name}` instead."
+            config;
+        })
+        config.nodes;
+
+    passthru.nodes = config.nodesCompat;
+  };
+}