Migrate transforms tests to pytest style (#1989)

Convert test_gaussian_blur.py, test_jigsaw.py and test_Solarize.py from
unittest.TestCase to plain pytest-style classes, following the pattern in
tests/loss/test_ibot_loss.py:

- Drop the unittest.TestCase base class and unittest import
- Replace manual width/height loops with pytest.mark.parametrize
- Convert the two deprecation tests to 'with pytest.warns(DeprecationWarning)';
  they previously called self.assertWarns without a with block, which asserts
  nothing, so they now actually verify the warning is raised

Transforms group of #1987.

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