JCBC-322 : Extended test coverage for the store, retrieve operations

This changeset adds more test coverage for the add, replace
and set operations which are currently not covered by the
underlying SPY tests.

Change-Id: Ie2ad5bd48dbea7ab09a76e3d4010355af5398231
Reviewed-on: http://review.couchbase.org/27167
Reviewed-by: Michael Nitschinger <michael.nitschinger@couchbase.com>
Tested-by: Michael Nitschinger <michael.nitschinger@couchbase.com>