Fix to tests makefile on Windows
Extract method: isAbsolutePath()
Add Win32-specific implementation of isAbsolutePath()

This just checks the second character for a colon, in the same way that
the POSIX one checks the first character for a slash.  As far as I am
aware, the two methods are equally valid on their respective platforms.
The Windows version won't work for 0-length pathnames, though.