Add test for gemspec files

[kaylareopelle]

#2113 revealed our recent changes to the files included in the Ruby agent unintentionally broke the NewRelic::Agent.require_test_helper public API. We have some tests that try to protect this from happening, but they didn't catch the problem in this case.

The lines of code to test are around the s.files assignment:

newrelic-ruby-agent/newrelic_rpm.gemspec

Lines 42 to 46 in 9fdb439

	reject_list = File.read('./.build_ignore').split("\n")
	file_list = `git ls-files -z`.split("\x0").reject { |f| reject_list.any? { |rf| f.start_with?(rf) } }
	build_file_path = 'lib/new_relic/build.rb'
	file_list << build_file_path if File.exist?(build_file_path)
	s.files = file_list

At a minimum, we need to test whether test/agent_helper.rb is included in the build.

[workato-integration]

https://issues.newrelic.com/browse/NR-140530

[ajesler]

I had a play this afternoon with spec'ing the test file inclusion, and this was where I got to.

Gem::Specification.load evals the gemspec file and builds the files list.

The raindow effect in the minitest output was an unexpected delight.

# frozen_string_literal: true
# test/new_relic/gem_file_test.rb
# $ TEST="test/new_relic/gem_file_test.rb" bundle exec rake test

require_relative '../test_helper'

class GemFileTest < Minitest::Test
  def test_the_test_agent_helper_is_shipped_in_the_gem_files
    agent_helper_file = 'test/agent_helper.rb'

    gem_spec_file_path = File.expand_path('../../../newrelic_rpm.gemspec', __FILE__)

    gem_spec = Gem::Specification.load(gem_spec_file_path)

    assert_equal('newrelic_rpm', gem_spec.name)
    assert_includes(gem_spec.files, agent_helper_file)
  end
end

[kaylareopelle]

This is great, @ajesler! Thank you! Glad you enjoyed minitest/pride! 🌈

I went ahead and added this, with one small tweak, to #2118 as a co-authored commit. Are you comfortable with that approach?

[ajesler]

Yep, that sounds great to me.