selftests: forwarding: Add a test for pedit munge tcp, udp sport, dport

Add a test that checks that pedit adjusts port numbers of tcp and udp
packets.

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>