MAINT: replace np.isinf(x) & (x>0) -> np.isposinf(x) to avoid RuntimeWarning

This fixes warnings of the type
```
scipy/special/_testutils.py", line 244, in check
    pinf_x = np.isinf(x) & (x > 0)
RuntimeWarning: invalid value encountered in greater
```
and is less complicated.