Document limitations of `./wpt serve` / "Running Tests from the Web" explicitly

[mbrodesser-Igalia]

¹ links to ² which mentions

"Not all tests can be executed in-browser, as some tests rely on automation (e.g. via testdriver.js) that is not available when running a browser in a normal user session."

One can try to reason which features aren't available in a normal user session, but listing them explicitly saves that effort.
This helps distinguishing actual bugs from technical limitations.

Footnotes

1. https://web-platform-tests.org/running-tests/from-local-system.html#via-the-browser ↩

2. https://web-platform-tests.org/running-tests/from-web.html ↩