Add E2E test of broken images on welcome page

What does this MR do?

This is less than ideal, but should give us some confidence that we'd catch a regression. At the very least, it would catch #34768 (closed) if the fixes were reverted.

I don't know of a foolproof way to wait for all images to load, so this sleeps for a second to give the DOM time to render and img tags, then loops until all the images found have a non-zero width (and can be fetched via HTTP).

Unfortunately, this doesn't account for SVGs. They're rendered as HTML, so there doesn't seem to be a way to check that they display properly via Selenium.

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content