Update snippets test to ignore visibility case

What does this MR do?

The recent work to convert Snippets to a Vue app included changing the hidden text used by a test from Private to private.

This MR updates the test so that it ignores case. It also removes the test from quarantine (the test needed an update so it doesn't need to be monitored in quarantine now that it's fixed).

Closes #209928 (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