selftests: virtio_net: add forgotten config options

One may use tools/testing/selftests/drivers/net/virtio_net/config
for example for vng build command like this one:
$ vng -v -b -f tools/testing/selftests/drivers/net/virtio_net/config

In that case, the needed kernel config options are not turned on.
Add the missed kernel config options.

Reported-by: Jakub Kicinski <[e-mail]>
Closes: https://lore.kernel.org/netdev/[e-mail]/
Reported-by: Matthieu Baerts <[e-mail]>
Closes: https://lore.kernel.org/netdev/[e-mail]/
Fixes: ccfaed04db5e ("selftests: virtio_net: add initial tests")
Signed-off-by: Jiri Pirko <[e-mail]>
Reviewed-by: Xuan Zhuo <[e-mail]>
Acked-by: Michael S. Tsirkin <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

tools/testing/selftests/drivers/net/virtio_net/config

···
-CONFIG_VIRTIO_NET=y
+CONFIG_BPF_SYSCALL=y
+CONFIG_CGROUP_BPF=y
+CONFIG_IPV6=y
+CONFIG_IPV6_MULTIPLE_TABLES=y
+CONFIG_NET_L3_MASTER_DEV=y
+CONFIG_NET_VRF=m
 CONFIG_VIRTIO_DEBUG=y
+CONFIG_VIRTIO_NET=y