CBQE-2361: fix swaprebalance_tunable test

Change-Id: If9ec5eabc6d34c8cfffc461e142c343e6d56ecd8
Reviewed-on: http://review.couchbase.org/40180
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>
Tested-by: Iryna Mironava <irynamironava@yandex.ru>