Rework isinf test case.
Rework isinf test case.

It's python execution led to various exceptions, which was not the
purpose of the test.

pombredanne/pythran,artas360/pythran,pombredanne/pythran,artas360/pythran,serge-sans-paille/pythran,artas360/pythran,hainm/pythran,pbrunet/pythran,serge-sans-paille/pythran,pbrunet/pythran,hainm/pythran,pombredanne/pythran,hainm/pythran,pbrunet/pythran