* stop using XDG_CACHE_HOME as home directory
XDG_CACHE_HOME is not a substitute for $HOME, see [1].
[1]: https://specifications.freedesktop.org/basedir-spec/basedir-spec-latest.html
* fix bats testing setup/teardown
since cmdutil.Homedir() would treat $XDG_CACHE_HOME as $HOME, deleting
$XDG_CACHE_HOME would wipe out previous kubens state. now that we're not
doing that, we need to make a real synthetic $HOME and clear it out so
that $HOME/.kube/kubens doesn't persist between runs.
