selftests: net: Create veth pair for testing in networkless kernel

Check if the netdev list is empty and create veth pair to be used for
feature on/off testing.
Remove the veth pair after testing is complete.

Signed-off-by: Abhinav Jain <jain.abhinav177@gmail.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20240821171903.118324-2-jain.abhinav177@gmail.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>