Move frontend tests to cudapy namespace (#558)

Follow up for
#523 (comment)

This moves some test files to the cudapy namespace and merges
`tests/test_extending` and `cudapy/test_extending.py`.

Author: brandon-b-miller

numba_cuda/numba/cuda/tests/complex_usecases.py renamed to numba_cuda/numba/cuda/tests/cudapy/complex_usecases.py

@@ -9,7 +9,7 @@
 from numba.cuda.testing import unittest, CUDATestCase
 from numba.cuda import types
 from numba import cuda
-from numba.cuda.tests.complex_usecases import (
+from numba.cuda.tests.cudapy.complex_usecases import (
     real_usecase,
     imag_usecase,
     conjugate_usecase,

numba_cuda/numba/cuda/tests/enum_usecases.py renamed to numba_cuda/numba/cuda/tests/cudapy/enum_usecases.py

@@ -20,7 +20,7 @@
     skip_on_cudasim,
     skip_on_standalone_numba_cuda,
 )
-from numba.cuda.tests.enum_usecases import (
+from numba.cuda.tests.cudapy.enum_usecases import (
     Color,
     Shape,
     Planet,