Add xctestrun snippet as a Release Toolkit action

[spencertransier]

While reviewing the Day One Apple Fastlane improvements with Jeremy, he suggested making a Release Toolkit action of this code:

    xctestrun_path = Dir.glob(File.join(build_products_path, '*.xctestrun')).select do |path|
      path.include?(options[:name])
    end.first

    if xctestrun_path.nil? || !File.exist?((xctestrun_path))
      UI.user_error!("Unable to find .xctestrun file at #{build_products_path}")
    end

It is a common set of code shared between many (all?) of the iOS apps, so potentially a good candidate for a Release Toolkit action.