Tests: use direct io for devices

[kdave]

https://lore.kernel.org/linux-btrfs/[email protected]/T/#u

With recent problems in CI where loop devices are not recognized after mkfs and fail mount we need to revisit how the devices are used. Outlined in the link, use only direct io for loop devices, add helpers that wrap that for device preparation as we can't use the automatic -o loop mount and audit all tests.