selftests: forwarding: Add test for mirror to gretap

Add a test for basic mirroring to gretap and ip6gretap netdevices.

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