[c++] Trivial name-shortens in unit-test code

[johnkerl]

Issue and/or context: In support of issue #2407 / [sc-51048].

Changes:

1st commit: I prefer longer/clearer names often, but, these are repeated and are causing unnecessary clang-format adjustments, particularly in PRs stacked on top of this.

2nd commit: another renaming. Not worth a separate PR IMO.

Notes for Reviewer:

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.61%. Comparing base (3bc119e) to head (c425bfd).
Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3125      +/-   ##
==========================================
+ Coverage   86.49%   86.61%   +0.12%     
==========================================
  Files          47       47              
  Lines        4924     4924              
==========================================
+ Hits         4259     4265       +6     
+ Misses        665      659       -6     

Flag	Coverage Δ
python	86.61% <ø> (+0.12%)	⬆️

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Components	Coverage Δ
python_api	86.61% <ø> (+0.12%)	⬆️
libtiledbsoma	∅ <ø> (∅)