Add Playwright tests for URL previews.

[Half-Shot]

As we're looking to improve this area in the coming weeks, we should have some tests that cover the current behaviour.

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)