**Summary**
Improve file-detection tests in preparation for additional work and bug
fixes.
**Additional Context**
- Add type annotations.
- Use mocks instead of `monkeypatch` in most cases and verify calls to
mock. This revealed a dozen broken tests, broken in that the mocks
weren't being called so a different code path than intended was being
exercised.
- Use `example_doc_path()` instead of hard-coded paths.
- Add actual test files for cases where they were being constructed in
temporary directories.
- Make test names consistent and more descriptive of behavior under
test.
