selftests: drv-net: resolve remote interface name

Find out and record in env the name of the interface which remote host
will use for the IP address provided via config.

Interface name is useful for mausezahn and for setting up tunnels.

Reviewed-by: Willem de Bruijn <willemb@google.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Link: https://patch.msgid.link/20250218225426.77726-2-kuba@kernel.org


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