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/run.nix

@@ -0,0 +1,51 @@
+{ config, hostPkgs, lib, ... }:
+let
+  inherit (lib) types mkOption;
+in
+{
+  options = {
+    passthru = mkOption {
+      type = types.lazyAttrsOf types.raw;
+      description = ''
+        Attributes to add to the returned derivations,
+        which are not necessarily part of the build.
+
+        This is a bit like doing `drv // { myAttr = true; }` (which would be lost by `overrideAttrs`).
+        It does not change the actual derivation, but adds the attribute nonetheless, so that
+        consumers of what would be `drv` have more information.
+      '';
+    };
+
+    run = mkOption {
+      type = types.package;
+      description = ''
+        Derivation that runs the test.
+      '';
+    };
+  };
+
+  config = {
+    run = hostPkgs.stdenv.mkDerivation {
+      name = "vm-test-run-${config.name}";
+
+      requiredSystemFeatures = [ "kvm" "nixos-test" ];
+
+      buildCommand =
+        ''
+          mkdir -p $out
+
+          # effectively mute the XMLLogger
+          export LOGFILE=/dev/null
+
+          ${config.driver}/bin/nixos-test-driver -o $out
+        '';
+
+      passthru = config.passthru;
+
+      meta = config.meta;
+    };
+
+    # useful for inspection (debugging / exploration)
+    passthru.config = config;
+  };
+}