Implement smoke tests

Recent issues such as #121 #122 #123 showed that our current test suite is inadequate and several edge cases are missing. This issue aims to outline a plan for implementing smoke tests to identify issues with the core APIs namely:

- `detect_document_text`
- `start_document_text_detection`
- `analyze_document`
- `start_document_analysis`
- `analyze_id`
- `analyze_expense`
- `start_expense_analysis`
- `get_result`

Each API should be callable with a Pillow image (lowest common-denominator) and the result should be visualizable.