Need to create a Rails engine which uses RSpec instead of Test::Unit and FactoryBot instead of fixtures? 

Here are the steps:

1. Create the engine:
  > rails plugin new icmoc_core -T --mountable --full --dummy-path=spec/test_app

  * -T tells the generator to skip Test::Unit
  * --mountable tells the generator that you want a namespaced engine
  * --full tells the generator that you want app and config directories
  * --dummy-path is the rails app that is generated for your tests. It's called test/dummy by default, but I never liked calling it that.

2. Add rails-rspec and factory_bot_rails as a dependencies in your Gemfile:
  
  source "http://rubygems.org"
  gemspec
  gem "rspec-rails", "~> 2.12.2"
  gem "factory_bot_rails", "~> 4.0"

Then bundle install.

3. Now edit your engine's lib/icmoc_core/engine.rb file to include rspec:
  module IcmocCore
    class Engine < ::Rails::Engine
      isolate_namespace GemName

      config.generators do |g|
        g.test_framework :rspec
        g.fixture_replacement :factory_bot, :dir => 'spec/factories'
      end
    end
  end

4. Run the rspec generator:

  > rails generate rspec:install

5. After creating some models and migrations, run the migrations for the test app and the test database:

  > bundle exec rake app:db:migrate
  > bundle exec rake app:db:test:prepare

Now you have a Rails engine configured to use RSpec.