test: expand limit_req_zone coverage

[ryepup]

Proposed changes

We fixed a bug in #105 and added a test, but the test didn't cover the original bug of analysis errors using the default DirectiveSources nor the differences between N+ and OSS.

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• [x] I have read the CONTRIBUTING document
• [x] If applicable, I have added tests that prove my fix is effective or that my feature works
• [x] If applicable, I have checked that any relevant tests pass after adding my changes
• [x] I have updated any relevant documentation (README.md)