backend: more deterministic test_delete_not_finished_workers
    
    Previously we relied on the fact that background action processor is
    able to start (== fork() and communicate with redis) within a very
    limited time period, and it often wasn't enough.
    
    Instead of increasing the period, make sure that we control the
    time.time() call in worker-manager so we 100% know how many loops happen
    in WorkerManager.run().  This requires us to limit the number of calls
    to time.time().