Adapted environs storage tests for Windows.

Made some minor adaptations to the storage-related tests in environs to work on Windows as well.
The only test still failing is TestPathRelativeToHome, which will work just fine once [this pull](https://github.com/juju/utils/pull/27) gets merge into the utils package and tilde expansion works on Windows as well.
I know that turns ugly; the problem is that, in this particular case, the input is a directory obtained from gocheck.MkDir() which automatically features backslashes. I assure you that the basic functionality is tested properly though; the only problem being that last /a joining which needed to be solved.