minitest_4.7.rb

gem 'minitest', '~> 4.7'

require 'minitest/autorun'
require 'open3'

class TestOutput < Minitest::Unit::TestCase
  def test_that_open3_works
    _,stdout,_ = Open3.popen3('rails -v')

    assert_match /Rails/, stdout.read.chomp
  end
end

minitest_5.x.rb

gem 'minitest', '~> 5'
require 'minitest/autorun'
require 'open3'

puts Minitest::VERSION
class TestOutput < Minitest::Test
  def test_that_open3_works
    _,stdout = Open3.popen2('rails -v')

    assert_match /Rails/, stdout.read.chomp
  end
end