tl2020 pretest begin

[Karl Berry]

    https://tug.org/texlive/pretest.html says \filedump is new in XeTeX, but 
    it was there in 2019. 

Thanks. Updating doc.

    (Aside: is there a good marker for TL'20 XeTeX 
    other than the version string?)

I guess there is no new primitive. If you find one, let me know.  (I
wish there was a way to "show all primitives" from an engine binary.) -k