test_helper.rb

class Test::Unit::TestCase
  def capture_io
    require 'stringio'
    
    orig_stdout, orig_stderr = $stdout, $stderr
    captured_stdout, captured_stderr = StringIO.new, StringIO.new
    $stdout, $stderr = captured_stdout, captured_stderr
    
    yield
    
    return captured_stdout.string, captured_stderr.string
  ensure
    $stdout = orig_stdout
    $stderr = orig_stderr
  end
end

class FooTest < Test::Unit::TestCase

  def test_io_sample
    stdout, stderr = capture_io do
      puts "x"
    end
    assert_equal "x\n", stdout
  end

end