Migrate test_setups 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_setups/test_setup_functions.py to pytest-style tests”

Details

• This PR migrates test_setup_functions.py from unittest.TestCase-based tests (openml.testing.TestBase) to pure pytest-style tests.
• It replaces the class-based TestSetupFunctions with module-level test functions and introduces a pytest fixture sklearn_extension() to provide the SklearnExtension instance previously created in setUp().
• The helper method _existing_setup_exists() is extracted as a standalone function that accepts sklearn_extension as a parameter.
• Tests that require the static cache directory now use the existing static_cache_dir fixture from conftest.py.
• Assertions continue to use assert and pytest.raises, consistent with the style already used in test_utils.py.
• Calls to TestBase._mark_entity_for_removal() and TestBase.logger are preserved for tracking entities uploaded to the test server during cleanup.
• No functional changes to the OpenML setups API are introduced; only the test implementation is refactored.