LU-11365 tests: test-framework::add() to load ldiskfs

when the tests run from in-tree build, then mkfs.lustre
can not rely on modprobe and ldiskfs needs to be loaded
explicitly. otherwise some tests like conf-sanity/55 fail.

Change-Id: Ic15f0eab8e483d5de9f63606433fbb26f754fd55
Signed-off-by: Alex Zhuravlev <bzzz@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/33140
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Wei Liu <sarah@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>