Create test for agent_helper in gem files

Co-authored-by: AJ Esler <aj@esler.co.nz>

test/new_relic/gemspec_files_test.rb

@@ -0,0 +1,20 @@
+# This file is distributed under New Relic's license terms.
+# See https://github.com/newrelic/newrelic-ruby-agent/blob/main/LICENSE for complete details.
+# frozen_string_literal: true
+
+require 'minitest/autorun'
+
+class GemspecFilesTest < Minitest::Test
+  def test_the_test_agent_helper_is_shipped_in_the_gem_files
+    skip if defined?(Rails::VERSION)
+
+    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