[c++] Pre-neatens before making unit-test helpers for variant-indexed dataframes

[johnkerl]

Issue and/or context: This is a line-count-reducing under-diff following on #2919 for issue #2407 / [sc-51048].

The intended Python and R API changes are all agreed on and finalized as described in #2407.

Changes:

Just some prep for my next step to unit-test #2916 and #2917, namely, we need libtiledbsoma-level support for variant-indexed dataframes (those whose dim is something other than a single int64).

Notes for Reviewer:

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.02%. Comparing base (ae3e417) to head (1738f7e).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2936      +/-   ##
==========================================
+ Coverage   89.87%   90.02%   +0.15%     
==========================================
  Files          38       38              
  Lines        3999     3999              
==========================================
+ Hits         3594     3600       +6     
+ Misses        405      399       -6     

Flag	Coverage Δ
python	90.02% <ø> (+0.15%)	⬆️

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

Components	Coverage Δ
python_api	90.02% <ø> (+0.15%)	⬆️
libtiledbsoma	∅ <ø> (∅)

[nguyenv]

Really minor comment but everything lgtm

[johnkerl]

Thanks @nguyenv !