Verify that user email is unique in test-unit rails

I a newbie in test-unit . I need to verify that email of the user is unique in the users table

require 'test_helper'

class UserTest < ActiveSupport::TestCase

  def setup
    @usuario = User.new
    @usuario.email="pepe@hotmail.com"
    @usuario.password="123456"
  end 

  test "Verify that user email is unique" do

  end    
end