tests: fix race condition between mkfs and udev
tests: fix race condition between mkfs and udev

Just after LVM volume is created, udev tries to analyze it. This
prevents mkfs from creating filesystem there.

QubesOS/qubes-core-admin,marmarek/qubes-core-admin,QubesOS/qubes-core-admin,QubesOS/qubes-core-admin,marmarek/qubes-core-admin,marmarek/qubes-core-admin