Run web platform tests in browsers

[MattiasBuelens]

Previously, we only ran the web platform tests (WPT) against Node using wpt-runner. This works fine, but it's not perfect:

• wpt-runner uses jsdom, and although jsdom does a really good job at implementing all the necessary web APIs, it's not a "real" browser.
• If the polyfill accidentally starts depending on a Node-only API, we may fail to spot it and break the polyfill for browser users. That would be pretty bad, after all this is still a web streams polyfill. 😛
• If we want to integrate with the built-in streams implementation of a web browser (Investigate using native streams #20), we really want to test that integration properly.

This PR adds the test infrastructure to also run WPT against actual browsers. I use Playwright to spin up a headless Chromium or Firefox browser, start a local web server, run all the tests and collect the results back inside Node. I tried to keep it as close as possible to how we run WPT on Node, to make it easier to understand and maintain.

Alternatives considered:

• Use wpt run as suggested in Running WPT #78. This is a Python tool used by the WPT project itself to run the tests. Although I eventually managed to get it to run, I decided not to go this route:
  • It's a very complex tool because it needs to support all of the test kinds in WPT, including Fetch tests with a HTTP server or Canvas tests with screenshots. For streams, most of this is not needed.
  • The polyfill does not pass all of the streams tests, so I need a way to have some expected failures. The tool provides wpt update-expectations, but I found it to not be granular enough.

[foolip]

Thanks @MattiasBuelens for outlining what issues you had with getting ./wpt run to work. I am not surprised that it was difficult to get working, but this is still useful feedback for the project. My takeaways:

• Getting ./wpt run to work at all the first time is too hard (I've heard this many times, and felt it myself)
• How to run a subset of tests or test types with ./wpt run is not very discoverable
• Using expectations with ./wpt run is not documented and is kind of broken. These code paths are used in Gecko's CI, but not using the ./wpt run frontend. I myself have never gotten it to work.

cc @gsnedders @jgraham @past FYI