Skipped OS-specific JujuHome tests in juju/osenv.

The JujuHomeLinux test was problematic considering that the Windows version of utils.Home() was being used when running the tests on Windows.
The JujuHomeWindows test patched a Linux-irrelevant environment variable and had no sense on the *nix-es

After further clarifications, it seems that I was wrong indeed APPDATA was the intended JujuHome path root.
The branch has been updated to only have the other OS's test skipped as they are irrelevant.

Thank you for your patience.