selftests: forwarding: Test mirror to gretap w/ bound dev

Test mirroring to a gretap and an ip6gretap netdevice with a bound
device, where the tunnel device and the bound device are in different
VRFs (an overlay / underlay configuration).

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