selftests: fib_rule_tests: Add DSCP selector connect tests

selftests: fib_rule_tests: Add DSCP selector connect tests

Test that locally generated traffic from a socket that specifies a DS
Field using the IP_TOS / IPV6_TCLASS socket options is correctly
redirected using a FIB rule that matches on DSCP. Add negative tests to
verify that the rule is not it when it should not. Test with both IPv4
and IPv6 and with both TCP and UDP sockets.

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/20240911093748.3662015-7-idosch@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>