selftests: fib_rule_tests: Add negative match tests

selftests: fib_rule_tests: Add negative match tests

The fib_rule{4,6} tests verify the behavior of a given FIB rule selector
(e.g., dport, sport) by redirecting to a routing table with a default
route using a FIB rule with the given selector and checking that a route
lookup using the selector matches this default route.

Add negative tests to verify that a FIB rule is not hit when it should
not.

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Link: https://patch.msgid.link/20240814111005.955359-4-idosch@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>