Increase test coverage

[jpmckinney]

https://coveralls.io/github/open-contracting/kingfisher-process?branch=django (2024: 81%)

The most missed lines (<80% >10 lines):

• [ ] process/management/commands/file_worker.py
• [ ] process/management/commands/compiler.py
• [ ] process/util.py
• [ ] process/management/commands/finisher.py
• [ ] process/management/commands/checker.py
• [ ] process/management/commands/release_compiler.py
• [ ] test_load.py had the comment (moved here) "TODO: test files with: bad encoding, nested data"
• [x] process/api.py
• [x] process/views.py

Once coverage is higher/while doing this, can also review quality/robustness of existing tests.

[jpmckinney]

cc @jakubkrafka