selftests: netfilter: synproxy test requires nf_conntrack

Otherwise, this test does not find the sysctl entry in place:

 sysctl: cannot stat /proc/sys/net/netfilter/nf_conntrack_tcp_loose: No such file or directory
 iperf3: error - unable to send control message: Bad file descriptor
 FAIL: iperf3 returned an error

Fixes: 7152303c ("selftests: netfilter: add synproxy test")
Signed-off-by: NPablo Neira Ayuso <pablo@netfilter.org>

tools/testing/selftests/netfilter/nft_synproxy.sh

@@ -23,6 +23,8 @@ checktool "ip -Version" "run test without ip tool"
 checktool "iperf3 --version" "run test without iperf3"
 checktool "ip netns add $nsr" "create net namespace"
 
+modprobe -q nf_conntrack
+
 ip netns add $ns1
 ip netns add $ns2