Fixed tests for version and agent to work on windows

This patch fixes tests for juju/agent and juju/version to work on windows, as well as ubuntu.
Here are a few of the things that had to change:
1) because Windows is not fully POSIX compliant, the file permissions checks have been disabled. os.Chmod() only works reliably on POSIX systems
2) Abstracted some error messages that differ based on OS. For example the "no such file or directory error"
3) used filepath.Join and FromSlash wherever needed
Due to the fact that Windows is not fully POSIX compliant, we skip this check. Instead of copying the code in another test exactly the same, except that line and to avoid code duplication, I just skip that one check on windows. I believe that the comment should be descriptive enough and no confusion will appear in the future due to this change. Thank you.