bug: avoid out of bounds (#74)

Add a test for out of bounds. In the wild this should not happen for
well-behaved servers. But there are ill-behaved servers too.

bug: propagate TLS errors (#73)

# Description

For integration into probe-cli, we need to propagate any errors raised
during TLS handshake.


While working on this commit, a few other improvements were made:

- the minivpn exported tracer accepts a transaction ID, for
compatibility with the array semantics used in the probe engine.
- minivpn was adapted to work with a downgraded version of a few
dependencies, to keep probe-cli dependency on go1.20.x:
- gateways (used for getting the default gateway for routes when using a
system tun device) is pinned to a go1.20-compatible version
  - package slices is changed in favor of 1.20 compat

# Checklist

* [x] I have read the contribution guidelines
* [x] Iff you changed code related to services, or inter-service
communication, make sure you update the diagrams in `ARCHITECTURE.md`.
* [x] Reference issue for this pull request:

test: run only integration tests in integration step

document TLS_NOVERIFY flag

expose stats in pinger

dial error