selftests: forwarding: add check_driver() helper

Add a helper to be used to check if the netdevice is backed by specified
driver.

Signed-off-by: Jiri Pirko <jiri@nvidia.com>
Reviewed-by: Benjamin Poirier <bpoirier@nvidia.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>