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

tools/testing/selftests/net/netfilter/packetdrill/conntrack_synack_reuse.pkt

@@ -30,5 +30,5 @@
 
 1.5 < S 2000:2000(0) win 32792 <mss 1000,nop,wscale 0, TS val 233 ecr 0,nop,nop>
 
-+0 `conntrack -L -p tcp --dport 8080 2>/dev/null | grep -q SYN_RECV`
++0 `conntrack -f $NFCT_IP_VERSION -L -p tcp --dport 8080 2>/dev/null | grep -q SYN_RECV`
 +0 `iptables -v -S INPUT | grep INVALID | grep -q -- "-c 0 0"`