migrate test cases for buffer fields

src/pystencils/__init__.py

src/pystencils/backend/kernelcreation/freeze.py

src/pystencils/backend/kernelcreation/iteration_space.py

src/pystencils/backend/typed_expressions.py

src/pystencils/config.py

src/pystencils/display_utils.py

src/pystencils/kernelcreation.py

src/pystencils/old/config.py

src/pystencils/old/cpu/kernelcreation.py

src/pystencils/old/gpu/kernelcreation.py

src/pystencils/old/kernelcreation.py

src/pystencils/old/typing/transformations.py

tests/test_buffer.py