tests/integ/basic: use export() in get_rootimg_checksum()
tests/integ/basic: use export() in get_rootimg_checksum()

volume.path and volume.export() refer to the same thing in lvm_thin and
'file', but not in file-reflink (where volume.path is the -dirty.img,
which doesn't exist if the volume is not started).

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