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-python.nix

@@ -166,6 +166,26 @@ rec {
           ${lib.optionalString (interactive) "--add-flags --interactive"}
       '');
 
+  evalTest = module: lib.evalModules { modules = testModules ++ [ module ]; };
+  runTest = module: (evalTest module).config.run;
+
+  testModules = [
+    ./testing/driver.nix
+    ./testing/interactive.nix
+    ./testing/legacy.nix
+    ./testing/meta.nix
+    ./testing/name.nix
+    ./testing/network.nix
+    ./testing/nodes.nix
+    ./testing/run.nix
+    ./testing/testScript.nix
+    {
+      config = {
+        hostPkgs = pkgs;
+      };
+    }
+  ];
+
   # Make a full-blown test
   makeTest =
     { machine ? null