nixos documentation: Suggest pkgs.testers.runNixOSTest (#351471)

### Testing outside the NixOS project {#sec-call-nixos-test-outside-nixos}

Outside the `nixpkgs` repository, you can instantiate the test by first importing the NixOS library,

Outside the `nixpkgs` repository, you can use the `runNixOSTest` function from

`pkgs.testers`:

```nix

let nixos-lib = import (nixpkgs + "/nixos/lib") { };

let pkgs = import <nixpkgs> {};

in

nixos-lib.runTest {

pkgs.testers.runNixOSTest {

  imports = [ ./test.nix ];

  hostPkgs = pkgs;  # the Nixpkgs package set used outside the VMs

  defaults.services.foo.package = mypkg;

}

```

`runTest` returns a derivation that runs the test.

`runNixOSTest` returns a derivation that runs the test.

## Configuring the nodes {#sec-nixos-test-nodes}