Migrate unittest.TestCase to pytest-style test classes

Several test files still use unittest.TestCase from before we adopted pytest. New tests are plain pytest-style classes (see tests/loss/test_ibot_loss.py for the pattern: no TestCase base, assert instead of self.assertEqual/self.assertTrue, pytest.mark.parametrize instead of manual test variants, pytest.mark.skipif instead of unittest.skipUnless). This migrates the rest.

Excluded from this issue: files being deleted by #1939 (API client removal) — tests/api/test_BitMask.py, tests/api/test_rest_parser.py, tests/api/test_utils.py, tests/api_workflow/mocked_api_workflow_client.py, tests/api_workflow/test_api_workflow_client.py. No point migrating tests that are about to disappear.

Split into one PR per group below — comment to claim a group, don't mix groups in one PR.

• Core — tests/core/test_Core.py
• Data — tests/data/test_LightlyDataset.py, tests/data/test_VideoDataset.py, tests/data/test_data_collate.py
• Embedding — tests/embedding/test_embedding.py
• Models — tests/models/test_ModelsBYOL.py, tests/models/test_ModelsMoCo.py, tests/models/test_ModelsNNCLR.py, tests/models/test_ModelsSimCLR.py, tests/models/test_ModelsSimSiam.py, tests/models/test_ModelUtils.py, tests/models/test_ProjectionHeads.py
• Model modules — tests/models/modules/test_ijepa_timm.py, tests/models/modules/test_masked_autoencoder.py, tests/models/modules/test_masked_autoencoder_timm.py, tests/models/modules/test_memory_bank.py
• Transforms — tests/transforms/test_gaussian_blur.py, tests/transforms/test_jigsaw.py, tests/transforms/test_Solarize.py
• Utils — tests/utils/test_debug.py, tests/utils/test_dist.py, tests/utils/test_io.py, tests/utils/test_version_compare.py

Mechanical conversion per file:

• class Foo(unittest.TestCase): → class Foo:
• setUp/tearDown → pytest fixture
• self.assertEqual(a, b) → assert a == b (same for assertTrue, assertListEqual, assertIsInstance, etc.)
• @unittest.skipUnless(cond, msg) → @pytest.mark.skipif(not cond, reason=msg)
• Drop import unittest where no longer needed

No behavior change expected — tests should still pass identically after conversion.