Draft: Add more integrations tests

The original idea was to add a benchmark, but I encountered some issues, so it's not very benchmarky yet (it does measurements, but does not report them).

I've put a fix for an small bug (Arti does not send END cell when it receive a TCP-FIN from the client) in there. It might be better cherry-picked as it's not exactly related, but I don't have time to properly test the fix (nor add unit test) right now (it's however caught by the "benchmark").

I don't think this is compatible with !89 (merged) as this other MR blocks connection to localhost, which is used for testing.