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'd use c.Check(..) here instead, as it makes it more obvious if the test fails.