tests(iroh-net): expose run_relay_server and option to skip cert verification to tests (#2145)

## Description

* Add a `test-utils` feature to `iroh-net`. If enabled, expose the
`test_utils` crate with `fn run_relay_server` publicly.
* Add option to `iroh_net::MagicEndpointBuilder` to skip relay server
SSL certificate verification. The option is behind a `#[cfg(any(test,
feature = "test-utils"))]`
* Add the same option to `iroh::NodeBuilder`, behind the same `cfg`
option
* Add a test to `iroh` which uses `run_relay_server` and then downloads
a blob by connecting to a node with only node id and relay URL (no
direct addresses)


## Change checklist

- [x] Self-review.
- [x] Documentation updates if relevant.
- [x] Tests if relevant.

Author: Frando