selftests: netfilter: add test case for conntrack helper assignment

check that 'nft ... ct helper set <foo>' works:
 1. configure ftp helper via nft and assign it to
    connections on port 2121
 2. check with 'conntrack -L' that the next connection
    has the ftp helper attached to it.

Also add a test for auto-assign (old behaviour).

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