[Unittest] Fix sporadic failure in test_dcp_cursor_dropping
[Unittest] Fix sporadic failure in test_dcp_cursor_dropping

Consider the case where the stream was closed before
any snapshot markers were sent.

Change-Id: I0b0f8add93b0874b9ca3f57ef28e252a34c66624
Reviewed-on: http://review.couchbase.org/53488
Tested-by: buildbot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>
Reviewed-by: Chiyoung Seo <fcffc23db27b19ee4ebba46695ee6b50e09216a2@couchbase.com>

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