設定

Gemfile

gem 'selenium-webdriver'

spec/rails_helper.rb


require 'selenium-webdriver'


# それぞれのUserAgentを持ったドライバを定義しておく
Capybara.register_driver :piyo_driver do |app|
  useragent = "piyo"
  capabilities = Selenium::WebDriver::Remote::Capabilities.chrome(
    chrome_options: {
      args: %W(headless disable-gpu window-size=1680,1050 user-agent=#{useragent})
    }
  )
  Capybara::Selenium::Driver.new(app, browser: :chrome, desired_capabilities: capabilities)
end

spec/features/piyo_spec.rb

require 'rails_helper'

# System Spec ができるようにまだしてない... :cry:

describe 'サンプルテスト' do
  it 'piyoというUserAgentでアクセスする', driver: :piyo_driver do
    # piyoというUserAgentが設定された状態でのテストができる
  end
end

参考

Configuring User Agents with Capybara + Selenium Webdriver « Plataformatec Blog
GitHubGist - Configure User Agent for selenium webdriver in Capybara
RSpec の feature spec でヘッドレス Chrome を使う - Speee DEVELOPER BLOG
ruby - How to use headless chrome with capybara and selenium - Stack Overflow
capybara/README.md at master · teamcapybara/capybara