Bring the tests README up to date #2

Source code/tests/README.md has fallen out of date:

• The title says "Tests for 4.1.3", but the tests cover much more than that version now.
• It says Node.js 16 or newer, while the main README and CONTRIBUTING.md say 18.
• It says to run npm install in ../agent only, but several tests also start the website, which needs npm ci in ../server too.

Done when: the file matches how the tests really run. You can check by following it in a fresh clone.