validation: add more test cases for private & slave propagations

We should test rootfs propagation modes, not only shared and unbindable,
but also slave and private.

Signed-off-by: Dongsu Park <dongsu@kinvolk.io>

Author: Dongsu Park

validation/linux_rootfs_propagation.go

@@ -22,6 +22,8 @@ func main() {
 
 	cases := []string{
 		"shared",
+		"slave",
+		"private",
 		"unbindable",
 	}