selftests: forwarding: Add a forwarding test for pedit munge dsfield

Add a test that runs packets with dsfield set, and test that pedit adjusts
the DSCP or ECN parts or the whole field.

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