Fix memory leak in engine_testapp when destroying cookie

We need to decrement the reference counter when we are done with a
cookie otherwise it won't actually be freed.

Change-Id: I35cc19f36a50a61e4be8f9701972c75695313fb3
Reviewed-on: http://review.couchbase.org/37977
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>