Tests fail under Python 3.13 due to use of undocumented unittest.makeSuite method: deprecated in Python 3.11, removed in Python 3.13

The test suite uses the method unittest.makeSuite which was never formally documented and never part of the Python standard library's official public API. This method was deprecated in Python 3.11 and removed in Python 3.13.

Update the test suite to use only documented and supported Python standard library methods.

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