CBQE-0: fix rebalance test

Change-Id: Ib44a2c8e99cb9ef5463cc9852a0858fc5b051b04
Reviewed-on: http://review.couchbase.org/31132
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>
Tested-by: Iryna Mironava <irynamironava@yandex.ru>