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/network.nix

@@ -0,0 +1,75 @@
+{ lib, nodes, ... }:
+
+with lib;
+
+
+let
+  machines = attrNames nodes;
+
+  machinesNumbered = zipLists machines (range 1 254);
+
+  nodes_ = forEach machinesNumbered (m: nameValuePair m.fst
+    [
+      ({ config, nodes, pkgs, ... }:
+        let
+          interfacesNumbered = zipLists config.virtualisation.vlans (range 1 255);
+          interfaces = forEach interfacesNumbered ({ fst, snd }:
+            nameValuePair "eth${toString snd}" {
+              ipv4.addresses =
+                [{
+                  address = "192.168.${toString fst}.${toString m.snd}";
+                  prefixLength = 24;
+                }];
+            });
+
+          networkConfig =
+            {
+              networking.hostName = mkDefault m.fst;
+
+              networking.interfaces = listToAttrs interfaces;
+
+              networking.primaryIPAddress =
+                optionalString (interfaces != [ ]) (head (head interfaces).value.ipv4.addresses).address;
+
+              # Put the IP addresses of all VMs in this machine's
+              # /etc/hosts file.  If a machine has multiple
+              # interfaces, use the IP address corresponding to
+              # the first interface (i.e. the first network in its
+              # virtualisation.vlans option).
+              networking.extraHosts = flip concatMapStrings machines
+                (m':
+                  let config = (getAttr m' nodes).config; in
+                  optionalString (config.networking.primaryIPAddress != "")
+                    ("${config.networking.primaryIPAddress} " +
+                      optionalString (config.networking.domain != null)
+                        "${config.networking.hostName}.${config.networking.domain} " +
+                      "${config.networking.hostName}\n"));
+
+              virtualisation.qemu.options =
+                let qemu-common = import ../qemu-common.nix { inherit lib pkgs; };
+                in
+                flip concatMap interfacesNumbered
+                  ({ fst, snd }: qemu-common.qemuNICFlags snd fst m.snd);
+            };
+
+        in
+        {
+          key = "ip-address";
+          config = networkConfig // {
+            # Expose the networkConfig items for tests like nixops
+            # that need to recreate the network config.
+            system.build.networkConfig = networkConfig;
+          };
+        }
+      )
+    ]);
+
+  extraNodeConfigs = lib.listToAttrs nodes_;
+in
+{
+  config = {
+    defaults = { config, name, ... }: {
+      imports = extraNodeConfigs.${name};
+    };
+  };
+}