selftests: netfilter: add zone stress test with colliding tuples

Add 20k entries to the connection tracking table, once from the
data plane, once via ctnetlink.

In both cases, each entry lives in a different conntrack zone
and addresses/ports are identical.

Expectation is that insertions work and occurs in constant time:

PASS: added 10000 entries in 1215 ms (now 10000 total, loop 1)
PASS: added 10000 entries in 1214 ms (now 20000 total, loop 2)
PASS: inserted 20000 entries from packet path in 2434 ms total
PASS: added 10000 entries in 57631 ms (now 10000 total)
PASS: added 10000 entries in 58572 ms (now 20000 total)
PASS: inserted 20000 entries via ctnetlink in 116205 ms

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