test_all: ignore TestMoveOntoItself on TestArchive

The archive backend returns the underlying objects, so a move onto the
same path isn't recognised as one and the file is copied onto itself.

fstest/test_all/config.yaml

@@ -725,6 +725,9 @@ backends:
      # with the parent backend having a different precision.
      - TestServerSideCopyOverSelf
      - TestServerSideMoveOverSelf
+     # The underlying objects returned aren't recognised as being at the
+     # destination path, so the move copies the file onto itself.
+     - TestMoveOntoItself
  - backend: "internxt"
    remote: "TestInternxt:"
    fastlist: false