Fix flaky snippet tests

What does this MR do?

Fixes transient failure that occurs when the test clicks 'Delete' button but doesn't wait for deletion to complete. Conditional wait was added to pause until pop-up window disappears and deleted snippet is no longer visible.

Closes #219422 (closed) and #220747 (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