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
If you're going to test it like this, I think you may as well just use `version.Current.Series`.

However, I think it would be preferable to have tests for both Linux and Windows that run on both Linux and Windows. This would mean not using `filepath`, and parameterising the test so it takes a series and expected path separator.