Fixing a sporadic test failure

- Wait for flusher to settle after compaction,
before checking for deleted expired items.

Change-Id: Ib82de197f2c33320b835fab9e68ae3a6c3da5c0f
Reviewed-on: http://review.couchbase.org/31589
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>