Migrate core tests to pytest style (#1990)

Convert tests/core/test_Core.py from unittest.TestCase to a plain
pytest-style class, following the pattern in tests/loss/test_ibot_loss.py:

- Drop the unittest.TestCase base class and unittest import
- Convert tearDown to an autouse yield fixture with identical cleanup
- Replace self.assert* calls with plain asserts
- Add type annotations to the helper methods (removes the file's three
  pre-existing mypy errors)

Core group of #1987.

Co-authored-by: fruet <90271657+gabrielfruet@users.noreply.github.com>