tests: extend test 131 to cover availability of the write-zeroes

tests: extend test 131 to cover availability of the write-zeroes

This patch contains test which minimally tests write-zeroes on top of
working discard.

The following checks are added:
* write 2 clusters, write-zero to the first allocated cluster
* write 2 cluster, write-zero to the half the first allocated cluster

Signed-off-by: Denis V. Lunev <den@openvz.org>
Reviewed-by: Alexander Ivanov <alexander.ivanov@virtuozzo.com>