MB-10040: fix test logic with expiration items

Change-Id: I2a3a262e37e601879f3ba89c2debe592764f6316
Reviewed-on: http://review.couchbase.org/33031
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>
Tested-by: Andrei Baranouski <andrei.baranouski@gmail.com>