test/suites/backup: Fixes issue with import testing with LVM
test/suites/backup: Fixes issue with import testing with LVM

BTRFS test removes the snapshot directory, but LVM test does not.

New storage driver uses directory presence as indication not to delete volume.

Signed-off-by: Thomas Parrott <6b778ce645fb0e3dde76d79eccad490955b1ae74@canonical.com>

lxc/lxd,atxwebs/lxd,atxwebs/lxd,lxc/lxd,Skarlso/lxd,hallyn/lxd,atxwebs/lxd,stgraber/lxd,hallyn/lxd,mjeanson/lxd,Skarlso/lxd,lxc/lxd,mjeanson/lxd,stgraber/lxd,atxwebs/lxd,hallyn/lxd,Skarlso/lxd,lxc/lxd,stgraber/lxd,stgraber/lxd,stgraber/lxd,stgraber/lxd,hallyn/lxd,hallyn/lxd,stgraber/lxd,lxc/lxd,Skarlso/lxd,hallyn/lxd,lxc/lxd,lxc/lxd,mjeanson/lxd