Here is how I solved a similar problem:
# rails_helper.rb
RSpec.configure do |config|
  config.before :example, perform_enqueued: true do
    @old_perform_enqueued_jobs = ActiveJob::Base.queue_adapter.perform_enqueued_jobs
    @old_perform_enqueued_at_jobs = ActiveJob::Base.queue_adapter.perform_enqueued_at_jobs
    ActiveJob::Base.queue_adapter.perform_enqueued_jobs = true
    ActiveJob::Base.queue_adapter.perform_enqueued_at_jobs = true
  end
  config.after :example, perform_enqueued: true do
    ActiveJob::Base.queue_adapter.perform_enqueued_jobs = @old_perform_enqueued_jobs
    ActiveJob::Base.queue_adapter.perform_enqueued_at_jobs = @old_perform_enqueued_at_jobs
  end
end
Then in specs we can use:
it "should perform immediately", perform_enqueued: true do
  SomeJob.perform_later  
end