server.nix

{
  network.description = "hc-web continuous integration server";

  jenkinsMaster =
    { config, pkgs, ... }:
    {
      # server config ...
    };
}

test.nix

import <nixpkgs/nixos/tests/make-test.nix> ({ pkgs, ... }: {
  name = "boot";
  machine = (import server.nix).jenkinsMaster;
  testScript = ''
    $machine->start;
    # ... lotsa tests ...
  '';
 }

To be honest, I don’t know how nixops evaluates the module configs you hand it, e.g. in the example from the manual:

$ nixops create ./trivial.nix ./trivial-vbox.nix -d trivial

There are two attribute sets of modules given, only one of them contains nixops-specific options (stuff from https://nixos.org/nixops/manual/#ch-options). I’d think nixops might do something like merge every attribute with the same name in all files you give it on the command line, as follows:

listOfAttributeModules:
import <nixpkgs/nixos> { 
  configuration = {
    imports = [ nixopsOptionDeclarations ] ++ listOfAttributeModules;
  };
}

The part where all nixops-specific options are imported is important.
That’s just my guess, to be sure you’d have to look at the nixops source.