selftests: tc-testing: add missing netfilter config

On a default config + tc-testing config build, tdc will miss
all the netfilter related tests because it's missing:
   CONFIG_NETFILTER=y

Signed-off-by: Pedro Tammela <pctammela@mojatatu.com>
Reviewed-by: Davide Caratti <dcaratti@redhat.com>
Acked-by: Jamal Hadi Salim <jhs@mojatatu.com>
Reviewed-by: Davide Caratti <dcaratti@redhat.com>
Link: https://lore.kernel.org/r/20240124181933.75724-2-pctammela@mojatatu.com


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