selftests: netfilter: packetdrill: avoid failure on HZ=100 kernel

packetdrill --ip_version=ipv4 --mtu=1500 --tolerance_usecs=1000000 --non_fatal packet conntrack_syn_challenge_ack.pkt
conntrack v1.4.8 (conntrack-tools): 1 flow entries have been shown.
conntrack_syn_challenge_ack.pkt:32: error executing `conntrack -f $NFCT_IP_VERSION \
-L -p tcp --dport 8080 | grep UNREPLIED | grep -q SYN_SENT` command: non-zero status 1

Affected kernel had CONFIG_HZ=100; reset packet was still sitting in
backlog.

Reported-by: Yi Chen <yiche@redhat.com>
Fixes: a8a388c ("selftests: netfilter: add packetdrill based conntrack tests")
Signed-off-by: Florian Westphal <fw@strlen.de>

Author: Florian Westphal

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

@@ -26,7 +26,7 @@
 
 +0.01 > R 643160523:643160523(0) win 0
 
-+0.01 `conntrack -f $NFCT_IP_VERSION -L -p tcp --dport 8080 2>/dev/null | grep UNREPLIED | grep -q SYN_SENT`
++0.1 `conntrack -f $NFCT_IP_VERSION -L -p tcp --dport 8080 2>/dev/null | grep UNREPLIED | grep -q SYN_SENT`
 
 // Must go through.
 +0.01 > S 0:0(0) win 65535 <mss 1460,sackOK,TS val 1 ecr 0,nop,wscale 8>