MB-8012 Fix to unit test failures in set/add/del/WithMeta ops.

We recently made a change in persisting a deletion so that we
put the current epoch timestamp of the deletion in the
expiration field of the key's meta data.

This change fixes the unit test failures due to the above change.

Change-Id: I15ffa887c5464c7185176173a68ba86a6a9833df
Reviewed-on: http://review.couchbase.org/27698
Reviewed-by: Jin Lim <jin@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung.seo@gmail.com>