[python] Use pathlib.Path.iterdir to count fragments

[nguyenv]

Issue and/or context:

This is the fourth of multiple changes pulled out from from #2883 to remove tiledb-py from unit tests

Changes:

Replace tiledb.VFS and tiledb.FragmentInfoList with pathlib.Path.iterdir to count fragments without relying on tiledb-py

[johnkerl]

🚢

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.32%. Comparing base (57b516e) to head (6310907).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3168      +/-   ##
==========================================
+ Coverage   83.21%   83.32%   +0.10%     
==========================================
  Files          51       51              
  Lines        5462     5462              
==========================================
+ Hits         4545     4551       +6     
+ Misses        917      911       -6     

Flag	Coverage Δ
python	83.32% <ø> (+0.10%)	⬆️

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Components	Coverage Δ
python_api	83.32% <ø> (+0.10%)	⬆️
libtiledbsoma	∅ <ø> (∅)