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 <[e-mail]>
Reviewed-by: Davide Caratti <[e-mail]>
Acked-by: Jamal Hadi Salim <[e-mail]>
Reviewed-by: Davide Caratti <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

tools/testing/selftests/tc-testing/config

···
 #
 # Core Netfilter Configuration
 #
+CONFIG_NETFILTER=y
 CONFIG_NETFILTER_ADVANCED=y
 CONFIG_NF_CONNTRACK=m
 CONFIG_NF_CONNTRACK_MARK=y