Work around a weird pgi bug in test/unit/math.c

pgi fails to compile math.c, reporting that `-INFINITY` in `pt_norm_expected[]`
is a "Non-constant" expression. A simplified version of this failure is:

```c
#include <math.h>

static double inf1, inf2 = INFINITY;  // no complaints
static double inf3 = INFINITY;        // suddenly INFINITY is "Non-constant"

int main() { }
```

```sh
PGC-S-0074-Non-constant expression in initializer (t.c: 4)
```

pgi errors on the declaration of inf3, and will compile fine if that line is
removed. I've reported this bug to pgi, but in the meantime I just switched to
using (DBL_MAX + DBL_MAX) to work around this bug.