CBQE-1054: fix autocompaction test

fix check_compaction_status method. Now it's using active tasks

Change-Id: I1bc78f3b46119ee7a75ade2fbde47195f4dda461
Reviewed-on: http://review.couchbase.org/24697
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>
Tested-by: Iryna Mironava <irynamironava@yandex.ru>