Fix failure in design_management_spec.rb

What does this MR do?

The design management test tries to change the style of an element on the design tab immediately after clicking the link to switch to the tab. This change waits for the tab to become active after clicking it.

Closes #35874 (closed)

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