selftests: nft_flowtable.sh: no need for ps -x option

Some ps commands (e.g. busybox derived) have no -x option. For the
purposes of hash calculation of the list of processes this option is
inessential.

Signed-off-by: Boris Sukholitko <boris.sukholitko@broadcom.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>