selftests: forwarding: Test neighbor updates when mirroring to gretap

Test that when a mirror to gretap or ip6gretap netdevice is configured,
changes to neighbors are reflected.

Signed-off-by: Petr Machata <petrm@mellanox.com>
Reviewed-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>