It caused by activating etc by RubyGems or Bundler each versions of Ruby.

I see. Tricky situation, of course.

I figured it out. test-unit requires fileutils, and old fileutils versions (shipped with old rubies) used to require etc. I proposed a PR to test-unit that maybe can avoid the fileutils require there, but I think another workaround would be to add gem 'fileutils', '>= 1.2.0' to the Gemfile, to ensure that a version including ruby/fileutils#34 is used.

@hsbt, @deivid-rodriguez Thanks, now we are at green.

@olleolleolle My PR to test-unit was merged and released, so I think you can drop the change now and CI should be green.

Actually fileutils is also loaded by mkmf.rb during compilation of the gem, so the workaround is still needed.

I reinstated the workaround, and then turned the commits in a single commit.

@olleolleolle Thanks. We will remove the additional dependency after drop to support Ruby 2.5 and 2.6.