MB-8372: slightly less data for swap tests

vm with 4Ge was out of memory

Change-Id: I48af5e9b5e7ca4f0b202a232f52d9981cd2e7ac9
Reviewed-on: http://review.couchbase.org/26743
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>
Tested-by: Andrei Baranouski <andrei.baranouski@gmail.com>