MB-100: minor fix in MemcachetestRunner

Change-Id: Iee9429eb0e6df6fda3c348a2ac7612911878b6eb
Reviewed-on: http://review.couchbase.org/25634
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>
Tested-by: Andrei Baranouski <andrei.baranouski@gmail.com>