selftests: forwarding: Add an skbedit priority selftest

Add a test that runs traffic through a port such that skbedit priority
action acts on it during forwarding. Test that at egress, it is classified
correctly according to the new priority at a PRIO qdisc.

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>