test: add test case for data path with slash

Signed-off-by: Robin Dong <sanbai@taobao.com>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>
test: modify _cleanup_devices() to umount loop device by correct order

When using _make_device like:

 _make_device $STORE/0/
 _make_device $STORE/0/d1
 _make_device $STORE/0/d2

the _cleanup_devices() will umount '$STORE/0/' first and then '$STORE/0/d1',
which will cause error message like:

 umount: /tmp/sheepdog/095/0/d1: not found
 umount: /tmp/sheepdog/095/1/d0: not found
 umount: /tmp/sheepdog/095/1/d1: not found
 umount: /tmp/sheepdog/095/2/d0: not found
 umount: /tmp/sheepdog/095/2/d1: not found

So we have to modify _cleanup_devices() to umount long path first.

Signed-off-by: Robin Dong <sanbai@taobao.com>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>