tests: (fdisk) make sure we use the same sizes for MD devices

tests: (fdisk) make sure we use the same sizes for MD devices

It seems on some kernels MD can return error if the devices in RAID
have different size

 # mdadm -q --create /dev/md8 --chunk=64 --level=0 --raid-devices=2 /dev/sda1 /dev/sda2
 mdadm: RUN_ARRAY failed: Unknown error 524

 # dmesg
 ...
 [ 1485.148435] md/raid0:md8: cannot assemble multi-zone RAID0 with default_layout setting
 [ 1485.152306] md/raid0: please set raid.default_layout to 1 or 2
 [ 1485.154050] md: pers->run() failed ...
 [ 1485.154104] md: md8 stopped.

Signed-off-by: Karel Zak <kzak@redhat.com>