[python/c++] Add SOMAGroup::is_relative and use in pytest unit tests

[nguyenv]

Issue and/or context:

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

Changes:

• Add SOMAGroup::is_relative in C++ and associated binding in Pybind11
• Replace tiledb-py is_relative test with new binding

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.19%. Comparing base (21d9e71) to head (92764fc).
Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3166   +/-   ##
=======================================
  Coverage   83.19%   83.19%           
=======================================
  Files          51       51           
  Lines        5458     5458           
=======================================
  Hits         4541     4541           
  Misses        917      917           

Flag	Coverage Δ
python	83.19% <ø> (ø)

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Components	Coverage Δ
python_api	83.19% <ø> (ø)
libtiledbsoma	∅ <ø> (∅)

[johnkerl]

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