Fix test failures

[nobu]

Fixes following failures:

• NoMethodError on ruby 2.4

  https://github.com/ruby/rdoc/runs/2565344070?check_suite_focus=true#step:5:64

  Error: test_remove_unparseable_CVE_2021_31799(TestRDocRDoc): NoMethodError: undefined method `children' for Dir:Class
  /home/runner/work/rdoc/rdoc/test/rdoc/test_rdoc_rdoc.rb:467:in `block in test_remove_unparseable_CVE_2021_31799'
       464:       end
       465:
       466:       assert_equal file_list, @rdoc.remove_unparseable(file_list)
    => 467:       assert_equal file_list, Dir.children('.')
       468:     end
       469:   end
       470:
  

• EINVAL on Windows

  https://github.com/ruby/rdoc/runs/2565343916?check_suite_focus=true#step:5:58

  Error: test_remove_unparseable_CVE_2021_31799(TestRDocRDoc): Errno::EINVAL: Invalid argument @ utime_failed - | touch evil.txt && echo tags
  D:/rubyinstaller-head-x64/lib/ruby/3.1.0/fileutils.rb:1142:in `utime'
  D:/rubyinstaller-head-x64/lib/ruby/3.1.0/fileutils.rb:1142:in `block in touch'
  D:/rubyinstaller-head-x64/lib/ruby/3.1.0/fileutils.rb:1139:in `each'
  D:/rubyinstaller-head-x64/lib/ruby/3.1.0/fileutils.rb:1139:in `touch'
  D:/a/rdoc/rdoc/test/rdoc/test_rdoc_rdoc.rb:463:in `block (2 levels) in test_remove_unparseable_CVE_2021_31799'
       460:     temp_dir do
       461:       file_list = ['| touch evil.txt && echo tags']
       462:       file_list.each do |f|
    => 463:         FileUtils.touch f
       464:       end
       465:
       466:       assert_equal file_list, @rdoc.remove_unparseable(file_list)
  

[olleolleolle]

Good context added in commit messages! Thanks for these, they're helpful in that they fix issues but also teach what they fix.