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

tools/testing/selftests/tc-testing/config

@@ -8,6 +8,7 @@ CONFIG_VETH=y
 #
 # Core Netfilter Configuration
 #
+CONFIG_NETFILTER=y
 CONFIG_NETFILTER_ADVANCED=y
 CONFIG_NF_CONNTRACK=m
 CONFIG_NF_CONNTRACK_MARK=y