Add simple browser test for mobile layout nav menu
user
```ruby
# frozen_string_literal: true

require 'rails_helper'

describe 'Hamburger navigation menu', type: :feature, js: true do
  let(:course) { create(:course) }

  it 'works when the window is narrow' do
    visit "/courses/#{course.slug}"
    expect(page).to have_content 'Help'
    page.current_window.resize_to(500, 500)
    expect(page).not_to have_content 'Help'
    find('.bm-burger-button').click
    expect(page).to have_content 'Explore'
    find('.bm-menu-active').click
    expect(page).not_to have_content 'Explore'
  end
end
```
assistant