Dynamic error pages in Rails with testing

application.rb

# in configure block
config.exceptions_app = self.routes

error.html.slim

h1 Error #{response.status}

p The resource you are trying to reach cannot be found.
p= link_to 'Click here to go back', :back

error_pages_test.rb

require 'test_helper'

# Assuming capybara and capybara-webkit are installed

class ErrorPagesTest < ActionDispatch::IntegrationTest

  # This controller needs to think the app is in production mode, or else the errors get swallowed.
  # But we don't want the rest of the test suite to behave as such?
  # This is hacky but I'm at a loss as to how to test it otherwise
  Rails.application.configure do
    config.consider_all_requests_local       = false
    config.action_dispatch.show_exceptions = true
  end

  test 'should get 404 if visiting nonexistent page' do
    visit '/nonexistent-page'
    assert page.status_code == 404
    assert page.has_content?('The resource you are trying to reach cannot be found.')
  end
  
  test 'should get 500 if error' do
    visit '/makes_500_error'
    assert page.status_code == 500
    assert page.has_content?('The resource you are trying to reach cannot be found.')
  end
  
  test 'should get 403 if forbidden' do
    visit '/no_u_cannot_haz'
    assert page.status_code == 403
  end

end

errors_controller.rb

class ErrorsController < ApplicationController
  def render(*args, &block)
    super 'error', *args, &block
  end

  def forbidden
    render status: 403
  end

  def not_found
    render status: 404
  end

  def internal_server_error
    render status: 500
  end
end

pages_controller.rb

class PagesController < ApplicationController
  # ...
  def no_u_cannot_haz
    redirect_to '/403'
  end
  # ..
end

routes.rb

# ...
get '/403', to: 'errors#forbidden'
get '/404', to: 'errors#not_found'
get '/500', to: 'errors#internal_server_error'
get '/makes_500_error', to: 'pages#template_with_error'
get '/no_u_cannot_haz', to: 'pages#no_u_cannot_haz'
# ...

template_with_error.html.slim

- raise "Hello Dolly"

Original concept is from https://mattbrictson.com/dynamic-rails-error-pages

I wanted to figure out how to test it.