tl2020 pretest begin

[Akira Kakuto]

Hi Joseph,

> (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

Sorry, they are described in https://tug.org/texlive/pretest.html  .
I think there is not a good marker for TL'20 XeTeX other than the version string.

Thanks,
Akira