MB-100: limit the number of checked items in upgrade tests

Change-Id: Ia5521accc181385fe0be7bb0c95749949b24ebfa
Reviewed-on: http://review.couchbase.org/24940
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>
Tested-by: Andrei Baranouski <andrei.baranouski@gmail.com>