test.rb

require 'minitest/autorun'

class Subject < Struct.new(:lol)
  def render_controls
    lol.button_to 'Vote'
  end
end

describe 'lolwot' do
  it "should generate a button" do
    template = Object.new

    tc     = self
    called = false

    template.extend Module.new {
      define_method(:button_to) do |arg|
        called = true
        tc.assert_equal "Vote", arg
        "THE_HTML"
      end
    }

    subject = Subject.new template

    subject.render_controls.must_equal "THE_HTML"
    assert called, 'button_to was called'
  end

  it "should do it with minitest/mock" do
    template = MiniTest::Mock.new
    template.expect :button_to, "THE_HTML", ["Vote"]

    subject = Subject.new template
    subject.render_controls.must_equal "THE_HTML"

    template.verify
  end
end