fcnal-test: kill hanging ping/nettest binaries on cleanup

On my box I see a bunch of ping/nettest processes hanging
around after fcntal-test.sh is done.

Clean those up before netns deletion.

Signed-off-by: Florian Westphal <fw@strlen.de>
Acked-by: David Ahern <dsahern@kernel.org>
Link: https://lore.kernel.org/r/20211021140247.29691-1-fw@strlen.de


Signed-off-by: Jakub Kicinski <kuba@kernel.org>