selftests: forwarding: Test mirror-to-gre w/ UL VLAN+802.1q

Test for "tc action mirred egress mirror" that mirrors to GRE when the
underlay route points at a vlan device on top of a bridge device with
vlan filtering (802.1q).

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