nixos: Document handleTest deprecation

        # (if it is a function).

        discoverTests (val { inherit system pkgs; })

      else val;

  /**

    Evaluate a test and return a derivation that runs the test as its builder.

    This function is deprecated in favor of runTest and runTestOn, which works

    by passing a module instead of a specific set of arguments.

    Benefits of runTest and runTestOn:

    - Define values for any test option

    - Use imports to compose tests

    - Access the module arguments like hostPkgs and config.node.pkgs

    - Portable to other VM hosts, specifically Darwin

    - Faster evaluation, using a single `pkgs` instance

    Changes required to migrate to runTest:

    - Remove any `import ../make-test-python.nix` or similar calls, leaving only

      the callback function.

    - Convert the function header to make it a module.

      Packages can be taken from the following. For VM host portability, use

      - `config.node.pkgs.<name>` or `config.nodes.foo.nixpkgs.pkgs.<name>` to refer

        to the Nixpkgs used on the VM guest(s).

      - `hostPkgs.<name>` when invoking commands on the VM host (e.g. in Python

        `os.system("foo")`)

    - Since the runTest argument is a module instead of a function, arguments

      must be passed as option definitions.

      You may declare explicit `options` for the test parameter(s), or use the

      less explicit `_module.args.<name>` to pass arguments to the module.

      Example call with arguments:

          runTest {

            imports = [ ./test.nix ];

            _module.args.getPackage = pkgs: pkgs.foo_1_2;

          }

    - If your test requires any definitions in `nixpkgs.*` options, set

      `node.pkgsReadOnly = false` in the test configuration.

   */

  handleTest = path: args:

    discoverTests (import path ({ inherit system pkgs; } // args));

  /** See handleTest */

  handleTestOn = systems: path: args:

    if elem system systems then handleTest path args

    else {};

        if elem system systems then runTest arg

        else {};

    })

    /** See https://nixos.org/manual/nixos/unstable/#sec-calling-nixos-tests */

    runTest

    /** See https://nixos.org/manual/nixos/unstable/#sec-calling-nixos-tests */

    runTestOn

    ;