selftests: forwarding: Add PCP match and VLAN match tests

Send packets with VLAN and PCP set and check that TC flower filters can
match on these keys.

Signed-off-by: Amit Cohen <amitc@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>