CBQE-0: shutdown all subtasks if test fails

Change-Id: I286556f7f551baa48b32f1bd0840da8eb1450465
Reviewed-on: http://review.couchbase.org/30704
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>
Tested-by: Iryna Mironava <irynamironava@yandex.ru>