selftests: forwarding: Move DSCP capture to lib.sh

selftests: forwarding: Move DSCP capture to lib.sh

dscp_capture_install() and dscp_capture_uninstall() are going to be
useful for a test added by a following patch, move them therefore to
lib.sh together with related helpers.

While doing so, change the rule preference from mere DSCP value to
DSCP+100 in order to support adding captures of packets with DSCP of 0.

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