class-method.rb

class Service
	def self.instance
		@instance ||= self.new
	end

	def initialize
	end

	def perform_action
		puts 'aciton performing'
	end
end

10.times do |variable|
	puts Service.instance.to_s
end

test.rb

require 'test_helper'
 
class PostsControllerTest < ActionController::TestCase
 
  # called before every single test
  def setup
    @post = posts(:one)
  end
 
  # called after every single test
  def teardown
    # as we are re-initializing @post before every test
    # setting it to nil here is not essential but I hope
    # you understand how you can use the teardown method
    @post = nil
  end
 
  test "should show post" do
    get :show, :id => @post.id
    assert_response :success
  end
 
  test "should destroy post" do
    assert_difference('Post.count', -1) do
      delete :destroy, :id => @post.id
    end
 
    assert_redirected_to posts_path
  end
 
end