selftests: nft_flowtable.sh: use /proc for pid checking

Some ps commands (e.g. busybox derived) have no -p option. Use /proc for
pid existence check.
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

@@ -288,11 +288,11 @@ test_tcp_forwarding_ip()
 
 	sleep 3
 
-	if ps -p $lpid > /dev/null;then
+	if test -d /proc/"$lpid"/; then
 		kill $lpid
 	fi
 
-	if ps -p $cpid > /dev/null;then
+	if test -d /proc/"$cpid"/; then
 		kill $cpid
 	fi