Ugly, but works.

some_test.rb

# ugly, but works

  def test_eval_display_node
    mock =  MiniTest::Mock.new.expect(:print, nil, ["H"])

    redefine_constant("STDOUT", mock) do

      ast = 72.times.collect { Mojikun::ThumbsUpNode.new }
      ast << Mojikun::DisplayNode.new

      @interpreter.evaluate(ast)
    end

    mock.verify
  end

  def redefine_constant(const, value)
    orig = eval(const).dup

    Object.send(:remove_const, const)
    Object.const_set(const, value)

    yield

    Object.send(:remove_const, const)
    Object.const_set(const, orig)
  end