PCBC-214: Fix expiration tests

Change-Id: I112f1ce04fffa786e8510b8955528675ffc80b3f
Reviewed-on: http://review.couchbase.org/25304
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>