Sync web-platform-tests PR 27272 into mozilla-central (this bug is closed when the sync is complete).

PR: https://github.com/web-platform-tests/wpt/pull/27272
Details from upstream follow.

b'Anne van Kesteren <[email protected]>' wrote:

WIP

@jakearchibald this is the playground I used to inspect various behaviors in browsers. To test scenarios such as how browsers validate 206 responses, how they deal with cross-origin redirects after the initial 206 response, etc. I need some way to describe these scenarios and get long-wav.py to handle them. Hardcoding it all would work, but that does not seem exactly elegant. Was wondering if you had ideas.