Clear last upr op in testsuite before adding latest op data

Not all of the upr fields are used for each message so it is
possible to have stale data sitting around that can cause tests
to verify behavior incorrectly. Similarly if no new message is
received after calling step then the old data will still be
sitting around.

Change-Id: I5736fc3c2835a5a8968bc94d4920ab65fa8b9468
Reviewed-on: http://review.couchbase.org/34571
Reviewed-by: Junyi Xie <junyi.couchbase@gmail.com>
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>