How to require file from `gem` which are not under `lib` directory?

I was so blinded, I already have path to file and able to get relative from it.

require 'pathname'
rubocop_path = Pathname.new(Gem.find_files('rubocop.rb').first).dirname
rubocop_path # => ...gems/rubocop-0.29.1/lib
require "#{rubocop_path}/../spec/support/cop_helper.rb"

I found a solution that I think is a little more syntactically elegant:

gem_dir = Gem::Specification.find_by_name("my_gem").gem_dir
require "#{gem_dir}/spec"