selftests: forwarding: Use OK instead of PASS in test output

It is easier to distinguish "[ OK ]" from "[FAIL]" than "[PASS]".

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Suggested-by: David Ahern <dsahern@gmail.com>
Cc: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>