Looks like minitest bundled with Ruby 1.9.x, not Test::Unit ([link to test-unit replacement gem docs](http://rubydoc.info/gems/test-unit/2.5.5/frames))

`require 'test/unit'` uses a compatibility layer to allow a seamless transition ([background](http://www.rubyinside.com/a-minitestspec-tutorial-elegant-spec-style-testing-that-comes-with-ruby-5354.html)):

> With Ruby 1.9, however, MiniTest entered the standard library. require 'test/unit' still works in Ruby 1.9 but it's provided through a compatibility layer on top of MiniTest, so if you're using require 'test/unit' in Ruby 1.9, you're really using MiniTest under the hood.