Fix test_numba: lazy import cuda to avoid CPU crash

Move 'from numba import cuda' from module level into the @gpu_test
method. numba-cuda v0.30.0 requires libcudart.so at import time,
which is not available on the CPU image.

Author: stevemessick

tests/test_numba.py

@@ -1,7 +1,7 @@
 import unittest
 
 import numpy as np
-from numba import jit, cuda
+from numba import jit
 
 from common import gpu_test
 
@@ -20,6 +20,8 @@ def go_fast(a): # Function is compiled to machine code when called the first tim
 
     @gpu_test
     def test_cuda_jit(self):
+        from numba import cuda
+
         x = np.arange(10)
 
         @cuda.jit