selftests: add script to stress-test nft packet path vs. control plane

Start flood ping for each cpu while loading/flushing rulesets to make
sure we do not access already-free'd rules from nf_tables evaluation loop.

Also add this to TARGETS so 'make run_tests' in selftest dir runs it
automatically.

This would have caught the bug fixed in previous change
("netfilter: nf_tables: do not skip inactive chains during generation update")
sooner.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>