sysctl: Add a selftest for handling empty dirs

Basic test to ensure that empty directories can be registered and that
they in turn can serve as a base dir for other registrations.

Add one test to the sysctl selftest module. It first registers an empty
directory under "empty_add" and then uses that as a base to register
another empty dir.
The sysctl bash script then checks that "empty_add" is present and that
there an empty directory within it.

Signed-off-by: Joel Granados <j.granados@samsung.com>
Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>