overlay: add test for persistent unique fsid

overlay: add test for persistent unique fsid

Test overlayfs fsid behavior with new mount options uuid=null/on
that were introduced in kernel v6.6:

- Test inherited upper fs fsid with mount option uuid=off/null
- Test uuid=null behavior for existing overlayfs by default
- Test persistent unique fsid with mount option uuid=on
- Test uuid=on behavior for new overlayfs by default

Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Reviewed-by: Zorro Lang <zlang@redhat.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>