CBQE-0: fix tearDown() in CompactionTests and call delBucket() function instead of exec() call

Change-Id: I196e247ebc040b5d629fa8c7c72abd58f67f3de4
Reviewed-on: http://review.couchbase.org/44420
Reviewed-by: Sangharsh Agarwal <sangharsh.agarwal@globallogic.com>
Tested-by: Sangharsh Agarwal <sangharsh.agarwal@globallogic.com>