Use a custom subdomain in an integration test

In an integration test ActionDispatch::IntegrationTest, you are not using Capybara, so you can't use Capybara.app_host=, but you can use the host! method to change the host url_for uses:

class MyTest < ActionDispatch::IntegrationTest
  def setup
    host! "admin.lvh.me"
  end

  test "#index" do
    visit my_url
  end
end