MB-9950 Wait for VB state snapshot before starting each unit test

This will make sure that all the storage files are created before
receiving the traffic.

Change-Id: I2f10e65928df56913b2b600a4d6c65fc89c5ab1d
Reviewed-on: http://review.couchbase.org/33158
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>