Pad stringification of exponent on systems that always use more than 2 digits

Microsoft chose to default to three digit exponents in the output of *printf
with the %e or %E conversion specifiers for doubles..

So sprintf a 0 to determine what the current system is doing and pad the
tests expected output strings by the appropriate amount of '0' characters.
Merge pull request #2 from veryrusty/windows/exponents

Pad stringification of exponent on systems that always use more than 2 digits