Add xctestrun snippet as a Release Toolkit action

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.

No file or test is named. Start by locating existing Release Toolkit actions and the shared iOS Fastlane uses of this snippet. Extract the .xctestrun lookup and shown missing-file error as reusable behavior, then verify it through the repository's existing tests; done means the common behavior is available as an action.