Adapt the repo to new l3build infrastructure

[hmenke]

Motivation for this change

There is still some stuff to do:

• Use Island of TeX Docker images for CI. This way we don't have to worry about TeX Live caching anymore. https://gitlab.com/islandoftex/images/texlive
• Check feasibility of l3build tag instead of our custom revisionfile
• Check feasibility of l3build upload instead of our custom GitHub Action
• Abstractions over lrbox. Right now the test files are a bit too LaTeX specific.

I'm leaving these for later. It's more important that we can start writing tests now.

• Boilerplate for macro-based tests (e.g. for pgfmath and softpaths)
• Convert https://github.com/pgf-tikz/pgf/tree/master/source/generic/pgf/testsuite into actually usable l3build tests.

Checklist

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• Code changes are licensed under GPLv2 + LPPLv1.3c
• Documentation changes are licensed under FDLv1.2

[muzimuzhi]

I'm also thinking of a set of per-format pre-processors so we can write tests for one format, e.g., \pgfpicture ... \endpgfpicture and run tests for all the supported formats.

[josephwright]

@muzimuzhi If you want to use the same tests for multiple formats, I'd suggest doing something like the d3... ones for LaTeX3: have a short wrapper .lvt file for each setup, and a .tex file which contains the 'payloads'.

[josephwright]

On the use of lrbox: this was a demo about one way to do catcode-dependent tests. As I showed, you could just \showoutput a suitable document. I used the LaTeX construct rather than a low-level one only because I've tried to stick to documented interfaces for the 'document level' syntax.

[josephwright]

@hmenke I looked at doing a pgf000 test (just loading), but I realise the current normalisation code doesn't like the pgf version strings. I'll look at sorting that, then will offer pgf000 and tikz000 (just checking loading). Beyond that, it would be handy to have a feeling for how you want tests dividing up. I'm sure I can manage a set of \pgf... ones based on the l3draw set.

[josephwright]

I've made a few comments, but I think it looks fine.

I might be tempted to rename the .lvt files to something like pgf-example and tikz-example.