fix remaining test suite

src/pystencils/boundaries/boundaryhandling.py

src/pystencils/codegen/target.py

src/pystencils/datahandling/datahandling_interface.py

src/pystencils/datahandling/serial_datahandling.py

src/pystencils/jit/gpu_cupy.py

tests/fixtures.py

tests/kernelcreation/test_functions.py

tests/kernelcreation/test_gpu.py

tests/kernelcreation/test_half_precision.py

tests/kernelcreation/test_index_kernels.py

tests/kernelcreation/test_iteration_slices.py

tests/runtime/test_boundary.py

tests/runtime/test_datahandling.py