[TEST] fix segfault in singleton test with cmake on macos-latest

[dbarker]

Fixes #3315

Changes

• add conditional to load the .dylib shared library files on macos in the singleton test

For significant contributions please make sure you have completed the following items:

• CHANGELOG.md updated for non-trivial changes
• Unit tests have been added
• Changes in public API reviewed

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.52%. Comparing base (b4328c0) to head (b2a5ffc).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3316   +/-   ##
=======================================
  Coverage   89.52%   89.52%           
=======================================
  Files         210      210           
  Lines        6522     6522           
=======================================
  Hits         5838     5838           
  Misses        684      684           

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[marcalff]

LGTM, thanks for the fix.