Fix to the sporadic failures on test_item_pager unit test
Fix to the sporadic failures on test_item_pager unit test

We just need to wait until the memory usage drops below high
watermark, to see if frequently referenced items are evicted or
not.

Change-Id: Ic766730403bf40b0478778bf53b8f5cf0b581154
Reviewed-on: http://review.couchbase.org/33194
Reviewed-by: Chiyoung Seo <fcffc23db27b19ee4ebba46695ee6b50e09216a2@couchbase.com>
Tested-by: Chiyoung Seo <fcffc23db27b19ee4ebba46695ee6b50e09216a2@couchbase.com>

daverigby/kv_engine,couchbase/ep-engine,jimwwalker/ep-engine,sriganes/ep-engine,couchbase/ep-engine,owendCB/ep-engine,couchbase/ep-engine,jimwwalker/ep-engine,jimwwalker/ep-engine,hisundar/ep-engine,teligent-ru/ep-engine,sriganes/ep-engine,teligent-ru/ep-engine,sriganes/ep-engine,daverigby/kv_engine,membase/ep-engine,jimwwalker/ep-engine,sriganes/ep-engine,teligent-ru/ep-engine,owendCB/ep-engine,owendCB/ep-engine,daverigby/kv_engine,daverigby/ep-engine,membase/ep-engine,hisundar/ep-engine,membase/ep-engine,daverigby/ep-engine,owendCB/ep-engine,daverigby/kv_engine,teligent-ru/ep-engine,couchbase/ep-engine,hisundar/ep-engine,hisundar/ep-engine,membase/ep-engine,daverigby/ep-engine,daverigby/ep-engine