selftests: fib_rule_tests: Add negative connect tests

The fib_rule{4,6}_connect tests verify 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
the given DS Field.

Add negative tests to verify that the FIB rule is not hit when the
socket specifies a DS Field that differs from the one used by the FIB
rule.

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Link: https://patch.msgid.link/20240814111005.955359-5-idosch@nvidia.com


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