selftest: netfilter: add test case for unreplied tcp connections

TCP connections in UNREPLIED state (only SYN seen) can be kept alive
indefinitely, as each SYN re-sets the timeout.

This means that even if a peer has closed its socket the entry
never times out.

This also prevents re-evaluation of configured NAT rules.
Add a test case that sets SYN timeout to 10 seconds, then check
that the nat redirection added later eventually takes effect.

This is based off a repro script from Antonio Ojea.

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