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/testScript.nix

@@ -0,0 +1,78 @@
+testModuleArgs@{ config, lib, hostPkgs, nodes, moduleType, ... }:
+let
+  inherit (lib) mkOption types;
+  inherit (types) either str functionTo;
+in
+{
+  options = {
+    testScript = mkOption {
+      type = either str (functionTo str);
+    };
+    testScriptString = mkOption {
+      type = str;
+      readOnly = true;
+      internal = true;
+    };
+
+    includeTestScriptReferences = mkOption {
+      type = types.bool;
+      default = true;
+      internal = true;
+    };
+    withoutTestScriptReferences = mkOption {
+      type = moduleType;
+      description = ''
+        A parallel universe where the testScript is invalid and has no references.
+      '';
+    };
+  };
+  config = {
+    withoutTestScriptReferences.includeTestScriptReferences = false;
+    withoutTestScriptReferences.testScript = lib.mkForce "testscript omitted";
+
+    testScriptString =
+      if lib.isFunction config.testScript
+      then
+        config.testScript
+          {
+            nodes =
+              lib.mapAttrs
+                (k: v:
+                  if v.virtualisation.useNixStoreImage
+                  then
+                  # prevent infinite recursion when testScript would
+                  # reference v's toplevel
+                    config.withoutTestScriptReferences.nodesCompat.${k}
+                  else
+                  # reuse memoized config
+                    v
+                )
+                config.nodesCompat;
+          }
+      else config.testScript;
+
+    defaults = { config, name, ... }: {
+      # Make sure all derivations referenced by the test
+      # script are available on the nodes. When the store is
+      # accessed through 9p, this isn't important, since
+      # everything in the store is available to the guest,
+      # but when building a root image it is, as all paths
+      # that should be available to the guest has to be
+      # copied to the image.
+      virtualisation.additionalPaths =
+        lib.optional
+          # A testScript may evaluate nodes, which has caused
+          # infinite recursions. The demand cycle involves:
+          #   testScript -->
+          #   nodes -->
+          #   toplevel -->
+          #   additionalPaths -->
+          #   hasContext testScript' -->
+          #   testScript (ad infinitum)
+          # If we don't need to build an image, we can break this
+          # cycle by short-circuiting when useNixStoreImage is false.
+          (config.virtualisation.useNixStoreImage && builtins.hasContext testModuleArgs.config.testScriptString && testModuleArgs.config.includeTestScriptReferences)
+          (hostPkgs.writeStringReferencesToFile testModuleArgs.config.testScriptString);
+    };
+  };
+}