selftests: netfilter: add selftest for ipip pmtu discovery with enabled connection tracking

Convert Christians bug description into a reproducer.

Cc: Shuah Khan <shuah@kernel.org>
Reported-by: Christian Perle <christian.perle@secunet.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Acked-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>