selftests: forwarding: Test gretap mirror with next-hop remote

Test mirror to a gretap and an ip6gretap netdevice such that the remote
address of the tunnel is reachable through a next-hop route.

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