test: assert that isinf returns -1 for negative infinity

Makes sure (and documents, in a sense) that isinf's declaration as returning
a bint preserves its C semantics.

tests/run/numpy_math.pyx

@@ -37,7 +37,8 @@ def test_fp_classif():
     assert not npmath.isnan(d_zero)
     assert not npmath.isnan(f_zero)
 
-    assert npmath.isinf(npmath.INFINITY)
+    assert npmath.isinf(npmath.INFINITY) == 1
+    assert npmath.isinf(-npmath.INFINITY) == -1
     assert npmath.isnan(npmath.NAN)
 
     assert npmath.signbit(npmath.copysign(1., -1.))