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>