selftests: virtio_net: add initial tests

Introduce initial tests for virtio_net driver. Focus on feature testing
leveraging previously introduced debugfs feature filtering
infrastructure. Add very basic ping and F_MAC feature tests.

To run this, do:
$ make -C tools/testing/selftests/ TARGETS=drivers/net/virtio_net/ run_tests

Run it on a system with 2 virtio_net devices connected back-to-back
on the hypervisor.

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