Improve tests

[caarmen]

• Use reverse in tests, to look up urls. This way if we have to change the url, we'll only have to do it in one place (urls.py).
• Add missing docstr to test.
• Split test_app into smaller files.
• Edit codecheck to fix issues.
  • Add --fix to automatically fix issues locally.
  • Add --exit-non-zero-on-fix so that the CI will fail if any issues were detected and fixed.