fixed format specified for test failure message

Change-Id: I38f8f1cc91eb5862871f6d8452c1007f4bc0e975
Reviewed-on: http://review.couchbase.org/44547
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>