Re-factor `system/requests/test_download.py`.
Re-factor `system/requests/test_download.py`.

- Replace `IMG_FILES` (tuple of `str`) with `ALL_FILES`, which is a tuple
  of info dictionaries for each file (including `path` and `content_type`)
- Factor the slice choices out of `test_download_partial` and
  `test_chunked_download_partial` and put them in `ALL_FILES`

googleapis/google-resumable-media-python,googleapis/google-resumable-media-python