selftests: forwarding: Test mirror-to-gre w/ UL VLAN

Test for "tc action mirred egress mirror" that mirrors to a gretap
netdevice whose underlay route points at a vlan device.

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