CBQE-1507: fix cbstats key tests

Change-Id: I9e4741629e4878f02661043b8104270afa80a4f9
Reviewed-on: http://review.couchbase.org/28050
Reviewed-by: Iryna Mironava <irynamironava@yandex.ru>
Tested-by: Iryna Mironava <irynamironava@yandex.ru>