selftests: net: add missing config for NF_TARGET_TTL

amt test uses the TTL iptables module:

  ip netns exec "${RELAY}" iptables -t mangle -I PREROUTING \
  	-d 239.0.0.1 -j TTL --ttl-set 2

Fixes: c08e8bae

 ("selftests: add amt interface selftest script")
Link: https://lore.kernel.org/r/20240131165605.4051645-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>