nixos/statsd: refactor test (#40554)

`statsd` is a daemon written in `node` to gather statistics over UDP.
The current test ensures that a port is open, but the basic
functionality isn't sufficiently tested.

This patch contains the following changes:

* Simplified port scanning (`waitForOpenPort` rather than `netcat` magic).

* Issue a TCP command to check the health of the `statsd` server.

* Simple script to check if `statsd` receives data over UDP and confirms
  the basic functionality of the TCP interface on `8126` for admin
  purposes.