The test requires quite some space(640G) to prepare a file on a separate
(not-cached) filesystem. Creating the file in rootfs is prone to (vague) errors
due to limited space.
To make sure that the test has all the required space available, create the file
on a separate disk
Signed-off-by: Michal Mielewczyk <michal.mielewczyk@huawei.com>