setupHomeTrash redirects HOME and XDG_DATA_HOME, but allTrashDirs also
walks readMountPoints() looking for <mount>/.Trash/<uid> and
<mount>/.Trash-<uid>, and no environment variable affects that walk. So
Count() and List() included whatever the developer had trashed on a
second filesystem, and Empty() deleted it for real: with an orphaned
file under a mount trash (no matching .trashinfo, so the count assert
still saw 3), TestEmptyClearsAll passed and removed the file.
Route the walk through a package-level mountPoints variable and have
setupHomeTrash swap in an empty list, restoring it via t.Cleanup.
Production behavior is unchanged.
bbedward