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 <chiyoung@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>