tests(multipath): add failing test

[Frando]

Description

This adds a simple test: one server that accepts connections, and then 3 clients that are created consecutively and each connect to the server, exchange data, close connection, close endpoint. After a client is finished the next client is created (new endpoint, new secret key).

The test comes in three variations that share their impl:

• test_one_server_two_clients_local_relay: uses a local relay
• test_one_server_two_clients_public_relay: uses the public relay
• test_one_server_two_clients_no_relay: doesn't use relays at all

The no relay and local relay versions pass for me. However the public relay one times out but only for the second client. The first one succeeds, the second connect attempt times out at the client.

Breaking Changes

Notes & open questions

Change checklist

• [ ] Self-review.
• [ ] Documentation updates following the style guide, if relevant.
• [ ] Tests if relevant.
• [ ] All breaking changes documented.
  • [ ] List all breaking changes in the above "Breaking Changes" section.
  • [ ] Open an issue or PR on any number0 repos that are affected by this breaking change. Give guidance on how the updates should be handled or do the actual updates themselves. The major ones are:
    • [ ] quic-rpc
    • [ ] iroh-gossip
    • [ ] iroh-blobs
    • [ ] dumbpipe
    • [ ] sendme