selftests: refactor get_netdev_name function

As pointed out by Michal Kubecek, getting the name
with the previous approach was racy, it's better
and easier to get the name of the device with this
patch's approach.

Essentialy the function doesn't need to exist
anymore as it's a simple 'ls' command.

Signed-off-by: Antonio Cardace <acardace@redhat.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>