This fixes the Root() returned by the backend when it has returned
fs.ErrorIsFile.
Before this change it returned a root which included the file path.
Because Root() was wrong this caused the detection of the file being
moved over itself check to fail.
This adds an integration test to check it for all backends.
See: https://forum.rclone.org/t/rclone-move-chunker-dir-file-chunker-dir-deletes-all-file-chunks/43333/
8b8156f7c3