Fix tests broken by recent updates to holdings directory

[rfrenchseti]

• Fixes #N/A
• Were any Django, import pipeline, or table_schema files modified? Y
  • If YES:
  • Database used (or new import): New import
  • FLAKE8 run on modified code: Y
  • All Django tests pass: Y
  • Code coverage run and still at 100%: Y
• Were any JavaScript or CSS files modified? N
  • If YES:
  • JSHINT run on all affected files: Y/NA
  • Tested on Chrome, Firefox, Safari, Edge, and iOS: Y/NA (Remember to test all browser sizes)
  • Tested for race conditions (with delayed API calls if applicable): Y/NA
• Was the documentation reviewed for necessary changes or additions? NA
  • About
  • Getting Started
  • FAQ
  • API Guide
  • Tooltips
  • Import/database schema

Description of changes:

Replaced remaining instances of OrderedDict to make future test debugging easier. Fixed two problems with tests caused by updated to Dropbox:

• Some Cassini "extras" JPG files were previously missing
• The Voyager documentation Image-Processing.txt was added

Known problems:

None