(Hopefully) resolve flaky tests

[dralley]

Mark tests that rely on global state as serial-execution-only

Assisted-By: claude-opus-4.6

📜 Checklist

• Commits are cleanly separated with meaningful messages (simple features and bug fixes should be squashed to one commit)
• A changelog entry or entries has been added for any significant changes
• Follows the Pulp policy on AI Usage
• (For new features) - User documentation and test coverage has been added

See: Pull Request Walkthrough

[ggainey]

In this case, it's not the orphan-cleanup that causes a race - it's the window between 369-404 where som eother test creating a repo will cause the assert to fail. Let's change the comment to be more-correct to avoid confusing Future Us

[ggainey]

Note that test_create_missing_repos will continue to occasionally fail with the "same content from different repos" deadlock. But these changes will make other tests more reliable, so it's definitely still worth doing.