Document server setup for the tests

[jrfnl]

The tests need a server to test various requests against. The code for this server is hosted and maintained in https://github.com/RequestsPHP/test-server

This server code was previously deployed to a Heroku instance. Once that became paid-only, we switched to Render last year.

The code changes for Render haven't been merged yet on the test-server repo and this whole setup with the test server isn't well documented.

There is an issue open about this in RequestsPHP/test-server#7, but we should probably also ensure that this test setup with the test-server repo is also documented in this repo.