Update suites to use github.com/juju/testing.FakeHomeSuite.

Update dependencies to the last github.com/juju/testing revision.
Create JujuOsEnvSuite to host the existing logic shared in 
BaseSuite and FakeJujuHomeSuite.

Remove the (Make)FakeHome code from juju: it is now placed in
github.com/juju/testing.

The remaining bits are mechanical changes.
I'm not convinced about this. cp might not be in /bin.
Perhaps OK as a workaround, but it would be better if we
didn't rely on any command that we don't need to in tests.

I'm currently writing a recursive file copy function which
we can replace this with, so perhaps leave it as /bin/cp
pending that.