testing: selftests: nft_flowtable.sh: use random netns names

[ Upstream commit b71b7bf ]

"ns1" is a too generic name, use a random suffix to avoid
errors when such a netns exists.  Also allows to run multiple
instances of the script in parallel.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Sasha Levin <sashal@kernel.org>