Revert "test: assert that isinf returns -1 for negative infinity"
Revert "test: assert that isinf returns -1 for negative infinity"

This reverts commit 24cf7c9671b45e891a6c35b4f576fff3dff5c00e.

According to the numpy documentation npy_isinf merely provides an
equivalent of C99 isinf, which in turn is only required to return a
non-zero value for a given infinite argument.

The original commit breaks the test when compiled with clang & libc++.

da-woods/cython,da-woods/cython,cython/cython,fabianrost84/cython,cython/cython,scoder/cython,ChristopherHogan/cython,scoder/cython,ChristopherHogan/cython,fabianrost84/cython,fabianrost84/cython,fabianrost84/cython,da-woods/cython,cython/cython,scoder/cython,da-woods/cython,fabianrost84/cython,scoder/cython,ChristopherHogan/cython,cython/cython