Added test 08-roundtrip, to test the entire gearman loop (neb module -> gearmand -> worker -> gearmand -> result_thread).

This test loads the neb module, initiates a burst of service checks and
collectes the results. It then checks the received results for missing
or duplicated entries.
If mod_gm_opt->max_jobs is reached, exit the worker process only after gearman_worker_work has finished, instead of exiting in get_job. Exiting prematurely causes the recently finished job to be not acknowledged, and the job gets executed once again.
disable roundtrip check for now

this check is designed to never end