[Unittest] Fix sporadic failure in test_item_pager
[Unittest] Fix sporadic failure in test_item_pager

This sporadic failure is because item pager doesn't
run due to inadequate memory pressure. Tweek the
test case so that the item pager is forced to run
if it hasn't already. This can be acheived by setting
mem_high_wat to a value less than mem_used.

Change-Id: I832b4e585f2a2ac23be81f92dabdc747fb8d52ab
Reviewed-on: http://review.couchbase.org/53187
Tested-by: buildbot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>
Reviewed-by: Chiyoung Seo <fcffc23db27b19ee4ebba46695ee6b50e09216a2@couchbase.com>

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