PYTHON-5781 Async split for test_network_layer.py

- Add test/asynchronous/test_network_layer.py as the async source of truth
  with AsyncUnitTest and _IS_SYNC = False
- Regenerate test/test_network_layer.py via synchro from the async source
- Register test_network_layer.py in synchro converted_tests (alpha order)
- Add "AsyncMock": "MagicMock" replacement after AsyncMockPool to avoid
  substring collision in translate_docstrings

Author: aclark4life