ApplicationController.current_userをstubを使用した場合

user_request_spec.rb

require 'rails_helper'

RSpec.describe "UserController", type: :request do
  describe "#index" do
    
    let(:user) { FactoryGirl.create(:user) }
    
    context 'success' do
      before do
        allow_instance_of(ApplicationController).to receive(:current_user).and_return(user)
        get user_path user
      end

      it { expect(response).to render_template :show }
    end
  end
end