tl2020 pretest begin

[Karl Berry]

Hi Denis,

    `tlmgr -gui &` then Actions œôöò Remove TeX Live 2020
    ...
    indeed, nothing has been uninstalled/removed.

I hope someone else can look at the GUI code. Sounds like it's not
invoking what needs to be invoked.

    By contrast, `tlmgr remove --all` worked as expected since everything
    was removed: symlinks and (almost) /home/bitouze/texlive/2020 content
    (surprisingly, two files `install-tl` and `texmfcnf.lua` are still there).

I changed tlmgr remove --all to also remove those two files. (Will be in
tonight's update.) In my test, that ended up now being able to remove
the top-level directory since there was nothing left. --thanks, karl.