tl2020 pretest begin

[Akira Kakuto]

Hi Joseph,

> https://tug.org/texlive/pretest.html says \filedump is new in XeTeX, but 
> it was there in 2019. (Aside: is there a good marker for TL'20 XeTeX 
> other than the version string?)

(1) Support of \input {...}
(2) Bug fix in the primitimve \pdfsavepos by Clerk Ma
(3) Fix of a potential bug (seg fault for incorrect input)
    in the primitive \XeTeXinterchartoks by Clerk Ma
(4) Allow fetching value from a token defined by \Umathchardef,
    to be compatible with LuaTeX, by Hironobu Yamashita

Thanks,
Akira