Wait for design thumbnail to load in QA test

What does this MR do?

After adding a design, wait for the design thumbnail to load before trying to interact with it.

Closes #35313 (closed)

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content