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
We don't tend to use underscores in var names (despite format_1_18Suite being in trunk). `configData118WithoutUpgradedToVersion` would be better.