Ufunc test refactor

refactored a bit the ufunc tests so that the long list of ufuncs in each class no longer is needed.

Also, added a test to int reciprocal avoiding 0 as a parameter (issue #757). In NumPy, its implementation casts an overflowing float to an int (1.0/0.0 -> +inf) which is undefined behavior in C. As numba may be using a different back-end than the compiler used to compile NumPy, and even different compiler options, it is possible to have different results in numba and NumPy in this case in the same machine,