I'd recommend stubbing out the call to the Resque worker and simply returning what you'd expect it to return. Then test the Resque worker code in a separate test.
If your goal is specifically to test the functionality of the modal window, you could write a test that simply makes the call to the Resque worker and use Capybara to determine if the modal shows up. In this example, you would not want to stub the Resque call - simply end the test after you've found the modal window appears.