Update pull mirroring test to use admin creds

What does this MR do?

Updates the test to use admin credentials as a workaround for a permissions bug. See #13769 (closed)

Ideally, we wouldn't use admin credentials for this test because it shouldn't need them. But this lets us keep the test running. When #13769 (closed) is fixed we can switch back to the default credentials.

Closes #33759 (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