engine/features/step_definitions/backoffice/mounting_steps.rb

Given /^the engine is mounted on a non standard path$/ do
  Rails.application.routes.draw do
    mount Locomotive::Engine => '/my-custom-path', :as => 'locomotive'
  end
end

Then /^I should be able to access the backend$/ do
  # Ensure we can access the backend
  visit '/my-custom-path'
  page.should have_content 'LocomotiveCMS'

  # Ensure we can update the homepage content
  click_link 'Home page'
  click_button 'Save'
  page.should have_content 'Page was successfully updated'

  # Reset the routes back to normal once we are done
  Rails.application.reload_routes!
end