Implement Playwright tests to ensure calls persist across room switches

[Half-Shot]

This introduces more Element Call tests, this time using a fake Element Call widget that does just enough to enable PiP. This should be useful enough to know if we're doing things like cleaning up the widgets appropriately.

This will be modified to fail on purpose to test the problem being fixed by #31314

Checklist

• I have read through review guidelines and CONTRIBUTING.md.
• Tests written for new code (and old code if feasible).
• New or updated public/exported symbols have accurate TSDoc documentation.
• Linter and other CI checks pass.
• I have licensed the changes to Element by completing the Contributor License Agreement (CLA)

[BillCarsonFr]

Magnifique 🥇

[t3chguy]

Looks like this test may be flaky based on the annotation left here by CI

[Half-Shot]

good spot. Looks like occasionally we get swapped back to the old room after clicking on a room in the room list, more to investigate.

[Half-Shot]

Put in an extra test on the UI that Element has picked up the video call before we switch rooms, which hopefully prevents it from doing that odd switching behaviour.