selftests: fib_rule_tests: Add DSCP selector match tests

Add tests for the new FIB rule DSCP selector. Test with both IPv4 and
IPv6 and with both input and output routes.

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Guillaume Nault <gnault@redhat.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://patch.msgid.link/20240911093748.3662015-6-idosch@nvidia.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>