CBQE:0 Grab cbcollect before cleanup in xdcr tests

Change-Id: I717c01897dc208fbea2366161da0084ea9eb3c91
Reviewed-on: http://review.couchbase.org/40644
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>
Tested-by: Aruna Piravi <aruna@couchbase.com>