[c++] Unit-test variant-indexed dataframes

[johnkerl]

Issue and/or context: This is iterative progress for issue #2407 / [sc-51048].

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

Changes:

Leverages #2939 and #2945 to unit-test variant-indexed soma dataframes at the libtiledbsoma level.

No shape-accessor logic is checked on this PR: see #2916.

No resize logic is checked on this PR: see #2917.

Notes for Reviewer:

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.02%. Comparing base (da76951) to head (096094f).
Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2944      +/-   ##
==========================================
+ 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%)	⬆️

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