Add tracing to testnet node startup

`startNode` in `Testnet.Runtime` is silent during the startup sequence.
It waits up to 45 seconds for the port to close, then up to 120 seconds for the socket to appear, with no output in between.
When something hangs or times out, there is nothing to indicate how far the startup got.

The function should accept a `Tracer` and emit progress messages at each stage, something like:

- "Waiting for port N to close..."
- "Port closed, starting node process (PID X)..."
- "Waiting for socket at PATH..."
- "Socket found, node ready."

PR #6346 removed some earlier tracing from this code path.
This issue is about reintroducing visibility in a structured way via a proper `Tracer` parameter.

**Context:** Originally noted by @carbolymer in https://github.com/IntersectMBO/cardano-node/pull/6346 review comments.