overlay/031: add tests of whiteouts in lowerdir and both dirs

overlay/031: add tests of whiteouts in lowerdir and both dirs

In overlay/031, it only cover the test case of whiteouts in
origined upper dir. This patch add two cases cover the other
two situations:

1) Lower origined dir have whiteouts;
2) Both upper and lower origined dirs have whiteouts (although
this case is pass now, still add this to cover all three kinds
of situations).

Signed-off-by: zhangyi (F) <yi.zhang@huawei.com>
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>