Migrate test_extensions to pytest

[anirudhsengar]

Metadata

• Reference Issue: Part of https://github.com/openml/openml-python/issues/1252
• New Tests Added: Yes (migrate existing tests to pytest style)
• Documentation Updated: No
• Change Log Entry: “Migrate test_extensions/test_functions.py to pytest-style tests”

Details

• This PR migrates test_functions.py from unittest.TestCase-based tests (openml.testing.TestBase) to pure pytest-style tests.
• It replaces the class-based TestInit with module-level test functions and introduces a small pytest fixture to manage the registration state of dummy extensions used in the tests.
• The new clean_test_extensions fixture calls the existing _unregister() helper before and after each test, ensuring the global extension registry is not polluted across tests.
• Assertions continue to use assert and pytest.raises, consistent with the style already used in test_utils.py.
• No functional changes to the OpenML extensions API are introduced; only the test implementation is refactored.