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

Test for "tc action mirred egress mirror" that mirrors to GRE when the
underlay route points at an 802.1d bridge and packet egresses through a
VLAN device.

Besides testing basic connectivity, this also tests that the traffic is
properly tagged.

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