Auto merge of #22809 - asajeffrey:wpt-add-fullscreen-baseline-reftest, r=asajeffrey

Add a reftest that entering fullscreen mode takes over the window

<!-- Please describe your changes on the following line: -->

Add a baseline reftest for fullscreen that ensures that going fullscreen actually uses the whole window. This uses a pref that requests that fullscreen mode takes over the window rather than the whole screen. The pref doesn't do anything right now, but is a bit of future-proofing for when we do have proper fullscreen.

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `___` with appropriate data: -->
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #22804.
- [X] There are tests for these changes

<!-- Also, please make sure that "Allow edits from maintainers" checkbox is checked, so that we can help you if you get stuck somewhere along the way.-->

<!-- Pull requests that do not address these steps are welcome, but they will require additional verification as part of the review process. -->

<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/22809)
<!-- Reviewable:end -->

0 files changed, 0 insertions, 0 deletions