Refs #17684: Enhances rdoc for ActiveJob assert_performed assertions

You're right about the global state change, but not showing how to enable is little help.

My feeling is that something like this is missing, maybe in test_helper:

    def perform_enqueued_jobs
      @old_perform_enqueued_jobs = queue_adapter.perform_enqueued_jobs
      queue_adapter.perform_enqueued_jobs = true
      yield
    ensure
      queue_adapter.perform_enqueued_jobs = @old_perform_enqueued_jobs
    end

Maybe this may be placed in test_adapter.

So one can code this test:

    perform_enqueued_jobs do
      assert_performed_jobs 1 do
        # ...
      end
    end