selftests: netfilter: fix packetdrill conntrack testcase

Some versions of conntrack(8) default to ipv4-only, so this needs to request
ipv6 explicitly, like all other spots already do.

Fixes: a8a388c2

 ("selftests: netfilter: add packetdrill based conntrack tests")
Reported-by: Jakub Kicinski <kuba@kernel.org>
Closes: https://lore.kernel.org/netdev/20240513114649.6d764307@kernel.org/
Signed-off-by: Florian Westphal <fw@strlen.de>
Link: https://lore.kernel.org/r/20240514144415.11433-1-fw@strlen.de
Signed-off-by: Jakub Kicinski <kuba@kernel.org>