selftests: forwarding: Test flower mirror to gretap

Add a test for mirroring to a gretap and an ip6gretap netdevices such
that the mirroring action is triggered by a flower match.

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