WiP: Made proper use of utils.Home() on Windows in the juju/osenv package.

Previously, if you called JujuHome() from the juju/osenv package, it would return a path with a different root than that of utils.Home() (which is much more widely used as a base along the project, if not everywhere now).

Now JujuHome() will return a path with a root consistent with the one of utils.Home(), preventing any possible conflicts which might have arisen from this.

The associated tests were also updated, OS-specific skippage being added as well.
I know, that was the problem.

Seeing as though the Linux implementation right above it uses utils.Home(), I'm guessing that jujuHomeWin was most likely implemented before a Windows version ofHome() was added to the utils package.
You can't really see any immediate repercussions of this in the tests as most of everything is patched beforehand; but in actual usage a "\Juju" folder will have been created/examined both in C:\Users\Whoever (the root provided by utils.Home() which is used in a lot of places) and in C:\Users\Whoever\Appadata; which might have lead to some big discrepancies later on.