Fix sporadic failure in unit tests when getting upr takeover estimates

The estimates might not be valid immediately because the backfill
task needs to bescheduled onto a background thread. As a result we
should wait for the estimate to be correct as opposed to just
checking it once.

Change-Id: I540cf76194e19018be934f43e097143b01df3e12
Reviewed-on: http://review.couchbase.org/37142
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>