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: 7152303cbec4 ("selftests: netfilter: add synproxy test")
Signed-off-by: Pablo Neira Ayuso <[e-mail]>

tools/testing/selftests/netfilter/nft_synproxy.sh

···
 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