selftests: forwarding: Test mirror-to-vlan

Test for "tc action mirred egress mirror" that mirrors to a vlan device.
- test_vlan() tests that the packets get mirrored
- test_tagged_vlan() tests that the mirrored packets have correct inner
  VLAN tag.

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