tests: force reloading partition table after setting partitioned loop dev
tests: force reloading partition table after setting partitioned loop dev

Apparently "losetup -P" doesn't always properly read partition table.
Force reload using blockdev --rereadpt.

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