refactor(local_janitor_spec.rb) stub out dir
refactor(local_janitor_spec.rb) stub out dir

Now that we've extracted the responsibility of finding and sorting the
filesystem, we have a nice boundary that we can test up to.

I've modified the #sift_trash tests so that they don't execute code
beyond the boundary. That is anything inside if BackupDir.

manheim/backupsss,manheim/backupsss