selftests: forwarding: Test mirror-to-gretap w/ UL team LACP

This tests mirror-to-gretap when an underlay packet path includes a team
device which is not in loadbalance mode, but in LACP mode. The test
manipulates LAG membership to achieve changes in txability, thus making
sure that a driver that offloads mirror-to-gretap doesn't just consider
upness of a device.

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