LU-8635 tests: Use wait_update to expire jobstats

Instead of arbitarily adjusting sleep count, use wait_update()
with maximum of (left+5) seconds to expire old jobstats.

Test-Parameters: trivial
Seagate-bug-id: MRP-2781
Signed-off-by: Ashish Purkar <ashish.purkar@seagate.com>
Signed-off-by: Emoly Liu <emoly.liu@intel.com>
Change-Id: I75e017fc75d35faf1754faddfed168c0fbf432fe
Reviewed-on: https://review.whamcloud.com/22699
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: James Nunez <james.a.nunez@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>