Test cleanup

Cool project! In trying things out locally, I noticed that there was a failing unit test around filter priority - I think the expectation should reflect what the production code does: sort higher priorities first.

Two other small cleanups:
- Avoid writing to zuul-core/moo on test runs.
- Eliminate inexact argument compiler warning.

I'm happy to split these out to separate PRs (or you can feel free to cherry-pick only the commits you want) if there are questions about any of the 3 commits. Just let me know if I should do anything differently.