[iops-unit-tests] Free timers in dtor

Change-Id: Ic135cd927a5912b7a3e770f92794f810f106f1d3
Reviewed-on: http://review.couchbase.org/28187
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>