The empty configuration test for the bottom module introduced as of https://github.com/nix-community/home-manager/pull/2323
is not cross platform. Specifically, it silently fails under a darwin environment due to
the configuration file not being generated at $XDG_CONFIG_HOME. This PR add cross platform support
by specifying the platform-dependent configuration directories to check. The expected unit test data
was also extracted to a separate file to differentiate between test data changes and
changes to the test itself.
