[c++] Unit-test the dataframe upgrader

[johnkerl]

Issue and/or context: As tracked on issue #2407 / [sc-51048].

Changes:

Unit-test the dataframe upgrader.

Notes for Reviewer:

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.16%. Comparing base (7018462) to head (3f5850b).
Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3139      +/-   ##
==========================================
+ Coverage   82.41%   83.16%   +0.75%     
==========================================
  Files          50       50              
  Lines        5203     5198       -5     
==========================================
+ Hits         4288     4323      +35     
+ Misses        915      875      -40     

Flag	Coverage Δ
python	83.16% <ø> (+0.75%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Components	Coverage Δ
python_api	83.16% <ø> (+0.75%)	⬆️
libtiledbsoma	∅ <ø> (∅)

[johnkerl]

Thanks @nguyenv !