select2_spec.rb

require 'rails_helper'

feature 'select2' do
  scenario 'select2', js: true do
    5.times { create(:disciplina) }
    visit new_disciplina_path
    execute_script("$('.disciplina_pre_requisitos .select2-input').val('ABC')")
    execute_script("$('.disciplina_pre_requisitos .select2-container').select2('open')")
    wait_for_ajax
    find('.select2-results li:first-child').click
    click_button 'Salvar'
  end
end

wait_for_ajax.rb

require 'timeout'

module WaitForAjax
  def wait_for_ajax
    Timeout.timeout(Capybara.default_wait_time) do
      loop do
        sleep 0.1
        break if finished_all_ajax_requests?
      end
    end
  end

  def finished_all_ajax_requests?
    page.evaluate_script('jQuery.active').zero?
  end
end

RSpec.configure do |config|
  config.include WaitForAjax
end