CBQE-0: add possibility to get traces if test failed

Change-Id: I12a68f3da48bb5783c993ef95aae9934cec5eb3f
Reviewed-on: http://review.couchbase.org/30166
Reviewed-by: Iryna Mironava <irynamironava@yandex.ru>
Tested-by: Iryna Mironava <irynamironava@yandex.ru>