Extend testsuit

This MR extends the test cases of pystencils.

Other changes made in this MR:

1. Usage of correct backends for the codegen instead of C Backend for all
2. Deletion of unusable function.
3. Correction of CUDA and OpenCL Array handler

pystencils/datahandling/pycuda.py

@@ -28,15 +28,15 @@ class PyCudaArrayHandler:
             return gpuarray.empty(shape, dtype)
 
     @staticmethod
-    def to_gpu(array: gpuarray):
+    def to_gpu(array):
         return gpuarray.to_gpu(array)
 
     @staticmethod
-    def upload(array: gpuarray, numpy_array):
+    def upload(array, numpy_array):
         array.set(numpy_array)
 
     @staticmethod
-    def download(array: gpuarray, numpy_array):
+    def download(array, numpy_array):
         array.get(numpy_array)
 
     def randn(self, shape, dtype=np.float64):