custom_plan.rb

require 'zeus/rails'

ROOT_PATH = File.expand_path(Dir.pwd)
ENV_PATH  = File.expand_path('spec/dummy/config/environment.rb',  __FILE__)
BOOT_PATH = File.expand_path('spec/dummy/config/boot', ROOT_PATH)
APP_PATH  = File.expand_path('spec/dummy/config/application', ROOT_PATH)

class CustomPlan < Zeus::Rails

  def test_environment
    Bundler.require(:test)

    ::Rails.env = ENV['RAILS_ENV'] = 'test'
    require APP_PATH

    $rails_rake_task = 'yup' # lie to skip eager loading
    ::Rails.application.require_environment!
    $rails_rake_task = nil

    $LOAD_PATH.unshift ".", "./lib", "./test", "./spec"
  end
end

Zeus.plan = CustomPlan.new

zeus.json

{
  "command": "ruby -rubygems -r./custom_plan -eZeus.go",

  "plan": {
    "boot": {
      "default_bundle": {
        "test_environment": {
          "test_helper": {"test": ["rspec", "testrb"]}
        }
      }
    }
  }
}