Fix rollback unit test to wait for updater to finish

The helper method used for shutting down view group should
wait for updater to finish before terminating the view group.

Change-Id: If3a8bd8eac88ece7ddf513b901647035fe58adee
Reviewed-on: http://review.couchbase.org/39152
Reviewed-by: Sarath Lakshman <sarath@couchbase.com>
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: buildbot <build@couchbase.com>