selftests: netfilter: use randomized netns names

Using ns0, ns1, etc. isn't a good idea, they might exist already.
Use a random suffix.

Also, older nft versions don't support "-" as alias for stdin, so
use /dev/stdin instead.
Signed-off-by: Florian Westphal <fw@strlen.de>
Acked-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>