Fix test error

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@62926 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

Author: znz

test/test_rdoc_markdown_test.rb

@@ -1,5 +1,5 @@
 # frozen_string_literal: true
-require 'minitest/autorun'
+require 'minitest_helper'
 require 'pp'
 
 require 'rdoc'