An example using Minitest::Mock, using the version (4.7) available in the Ruby standard library.

roro.rb

class Roro
  def initialize(host, speakers = [])
    @host = host
    @speakers = speakers
  end

  def start
    @host.speak
  end
end

test_roro.rb

require 'minitest/autorun'
require 'minitest/pride'

require_relative 'roro'

class TestRoro < MiniTest::Unit::TestCase
  def setup
    @mock = MiniTest::Mock.new
    @mock.expect :speak, 'Welcome to roro!'
  end

  def test_start_returns_welcome_message
    @roro = Roro.new(@mock)
    assert_equal 'Welcome to roro!', @roro.start
    @mock.verify
  end
end