[c++] Fix a compiler warning

[johnkerl]

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

Changes:

Fix a compiler warning:

/home/ubuntu/git/single-cell-data/TileDB-SOMA/libtiledbsoma/test/common.cc: In function ‘std::unique_ptr<ArrowSchema> helper::create_index_cols_info_schema(const std::vector<DimInfo>&)’:
/home/ubuntu/git/single-cell-data/TileDB-SOMA/libtiledbsoma/test/common.cc:156:26: warning: comparison of integer expressions of different signedness: ‘size_t’ {aka ‘long unsigned int’} and ‘int64_t’ {aka ‘long int’} [-Wsign-compare]
  156 |     for (size_t i = 0; i < schema->n_children; ++i) {

Notes for Reviewer:

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.50%. Comparing base (208c0ab) to head (5e97880).
Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3310      +/-   ##
==========================================
+ Coverage   85.37%   85.50%   +0.12%     
==========================================
  Files          52       52              
  Lines        5499     5506       +7     
==========================================
+ Hits         4695     4708      +13     
+ Misses        804      798       -6     

Flag	Coverage Δ
python	85.50% <ø> (+0.12%)	⬆️

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

Components	Coverage Δ
python_api	85.50% <85.71%> (+0.12%)	⬆️
libtiledbsoma	∅ <ø> (∅)

[bkmartinjr]

I don't mean to nitpick, but:

• do we encourage use of C-style (legacy) casts, vs say a static_cast<>?
• if schema->n_children is signed, alternative might just be to make i signed?
  either works for me...

[johnkerl]

Thanks @bkmartinjr !

[bkmartinjr]

approving, but left comments on possible improvement