rfctr(doc): spruce up test_doc.py (#3024)
**Summary**
In preparation for adding more tests related to image extraction,
improve the `partition_doc()` test suite:
- Remove redundant DOCX -> DOC file conversions on most tests.
- Add type annotations to type-check clean on strict mode.
- Improve test names.
- Simplify tests where possible.
- Remove one duplicated test
Speed was roughly doubled: 24 tests in 20s -> 23 tests in 8s.
