BigInteger: make it so you can do $min->random($max)

...and $min->randomPrime($max) as well
It should already do this thanks to this code:

```php
        if (!$compare) {
            return $this->_normalize($min);
        } else if ($compare < 0) {
            // if $min is bigger then $max, swap $min and $max
            $temp = $max;
            $max = $min;
            $min = $temp;
        }
```

Anyway I've updated the unit test accordingly. Also, see https://github.com/phpseclib/phpseclib/pull/373#discussion_r13787107