[c++] Update dataframe unit-test writes in prep for polytype domainish accessors

[johnkerl]

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

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

Changes:

Split-out underneath #3012

Notes for Reviewer:

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.79%. Comparing base (6f723d4) to head (bf78157).
Report is 1 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #3017       +/-   ##
===========================================
+ Coverage   61.10%   89.79%   +28.69%     
===========================================
  Files          95       40       -55     
  Lines       12382     4096     -8286     
  Branches      781        0      -781     
===========================================
- Hits         7566     3678     -3888     
+ Misses       4625      418     -4207     
+ Partials      191        0      -191     

Flag	Coverage Δ
libtiledbsoma	?
python	89.79% <ø> (+0.14%)	⬆️

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Components	Coverage Δ
python_api	89.79% <ø> (+0.14%)	⬆️
libtiledbsoma	∅ <ø> (∅)