overlay/040: clean up properly after setting immutable

Test program expects only immutable on lower layer (test failure),
but does not expect the immutable file to be on the upper layer.
The later case is actually what *should* happen, except overlayfs
didn't properly implement this case yet (but is now in the works).

Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>