tests: Ensure that loopback devices and zfs devices are cleaned up

tests: Ensure that loopback devices and zfs devices are cleaned up

ZFS file systems are not unmounted using umount, but instead by exporting
them. So export the ZFS file system that has the same label as the one that
was created during the test, if such one exists. This is required to delete
the loopback device that uses the ZFS image file. Otherwise the added code
to delete all loopback devices setup during the test run will never be able
to finish because the loopback device can not be deleted while in use.

Signed-off-by: Glenn Washburn <development@efficientek.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>