Convert files under `source/generic/pgf/testsuite/mathtest` into real tests

[muzimuzhi]

Brief outline of the proposed feature

Weeks ago I found the directory source/generic/pgf/testsuite/mathtest, which contains tests for fpu and luamath parser.

Maybe these tests can be added to CI, if the required efforts are affordable.

[muzimuzhi]

Sorry recently I feel down more often so am not so active and responsive here. (has nothing to do with tex nor pgf)

[hmenke]

The pgfmathtestsuite.tex just prints a bunch of stuff for visual inspection and unittest_luamathparser.tex doesn't work because it tries to test functions that are not implemented. But it's a good starting point to write proper tests with l3build.

[josephwright]

One could take the l3draw tests and use a bit of search-and-replace to repurpose many of them: most of the API is very similar to pgf. I might find a little time for that if it helps.