[tex4ht] [bug #345] regression testing is needed

[Karl Berry]

Hi Deimi and all,

    My idea is to adapt l3build system for tex4ht needs.

I think using l3build for test infrastructure is all to the good.
But we don't really want/need a copy of build.lua in the repo, do we?
Did you have to modify it? I can't tell if the differences from the
distributed version are just different versions, or something you needed
to do.

I see it somehow avoids testing the date line? If so, that alone is
an excellent reason to use it :).

Thanks much,
Karl