Fix checkpoint unit test caused by expecting wrong num of cursors

We put all of the cursors in the same structure now so we will
always have at least one cursor since we never remove the
persistence cursor.

Change-Id: Iea9e559ea72e200d71c36fcc20bd5b67c7435d10
Reviewed-on: http://review.couchbase.org/41559
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>