Migrate test_openml to pytest

[anirudhsengar]

Metadata

• Reference Issue: Part of https://github.com/openml/openml-python/issues/1252
• New Tests Added: Yes (migrated existing tests to pytest style)
• Documentation Updated: No
• Change Log Entry: "Migrate tests/test_openml to pytest-style tests"

Details

• This PR migrates all tests in test_openml from unittest.TestCase-based tests (openml.testing.TestBase) to pure pytest-style tests.
• Files migrated:
  • test_api_calls.py: Converted TestConfig class to standalone pytest functions (test_too_long_uri, test_retry_on_database_error)
  • test_config.py: Converted TestConfig and TestConfigurationForExamples classes to standalone pytest functions
  • test_openml.py: Converted TestInit class to standalone pytest function (test_populate_cache)
• Key changes:
  • Removed class wrappers inheriting from openml.testing.TestBase
  • Replaced unittest.skipIf with pytest.mark.skipif
  • Replaced self.assertRaisesRegex with pytest.raises context manager
  • Replaced self.assertDictEqual with plain assert statements
  • Replaced self.workdir (from TestBase) with pytest fixtures (tmp_path, workdir)
  • Changed unittest.mock imports to from unittest import mock for consistency
  • Updated tests to work with autouse fixtures from conftest.py (with_server, with_test_cache)
• Assertions now consistently use plain assert and pytest.raises, matching the style already used in test_utils/.
• No functional changes to the OpenML API are introduced; only test implementation is refactored.
• All 20 tests pass (2 skipped due to platform-specific requirements for Linux/non-Windows).