selftests: nft_flowtable.sh: wait for specific nc pids

selftests: nft_flowtable.sh: wait for specific nc pids

Doing wait with no parameters may interfere with some of the tests
having their own background processes.

Although no such test is currently present, the cleanup is useful
to rely on the nft_flowtable.sh for local development (e.g. running
background tcpdump command during the tests).

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