NCBC-388: Test case improved, first add a key and then verify that Store() should fail if we try to add it again.

Change-Id: If534ca3d8e6b22817ccdcf28c291000a1fcd9759
Reviewed-on: http://review.couchbase.org/33437
Reviewed-by: Jeffry Morris <jeffrymorris@gmail.com>
Reviewed-by: Saakshi Manocha <saakshi.manocha@globallogic.com>
Tested-by: Saakshi Manocha <saakshi.manocha@globallogic.com>