selftests: nft_flowtable.sh: monitor result file sizes

When running nft_flowtable.sh in VM on a busy server we've found that
the time of the netcat file transfers vary wildly.

Therefore replace hardcoded 3 second sleep with the loop checking for
a change in the file sizes. Once no change in detected we test the results.

Nice side effect is that we shave 1 second sleep in the fast case
(hard-coded 3 second sleep vs two 1 second sleeps).
Acked-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Boris Sukholitko <boris.sukholitko@broadcom.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

tools/testing/selftests/netfilter/nft_flowtable.sh

@@ -286,7 +286,15 @@ test_tcp_forwarding_ip()
 	ip netns exec $nsa nc -w 4 "$dstip" "$dstport" < "$nsin" > "$ns1out" &
 	cpid=$!
 
-	sleep 3
+	sleep 1
+
+	prev="$(ls -l $ns1out $ns2out)"
+	sleep 1
+
+	while [[ "$prev" != "$(ls -l $ns1out $ns2out)" ]]; do
+		sleep 1;
+		prev="$(ls -l $ns1out $ns2out)"
+	done
 
 	if test -d /proc/"$lpid"/; then
 		kill $lpid