Use lua-unicode-math in tests instead of unicode-math

[mbertucci47]

(This PR is not meant to be merged, at least not as-is. I also opened an issue to track this; see #1266.)

For the purposes of experimentation and speeding up the test suite, I tried changing every instance of \usepackage{unicode-math} in the test files with \usepackage{lua-unicode-math}\tagpdfsetup{math/mathml/luamml/load} and, where necessary, \usepackage{fontspec}. Some tests had no change, but many did have changes in the resulting mathml. I'm opening this PR just to provide a simple way to view the diffs of the xml files. Some changes may be expected, for example the several tests that error because of the different timing of when lua-unicode-math vs. unicode-math defines the symbols (immediately vs. at begindocument). I adjusted a few tests to avoid this but others produce confusing errors like MnSymbol and fdsymbol. I am hoping this will be useful to those who can look at the diffs and decide if the changes are expected or not, so @zauguin and @davidcarlisle I'd guess.

A few other observations:

• The prime character ′ in mathml goes from <mo lspace="0" rspace="0">′</mo> to <mi mathvariant="normal">′</mi>. Same for !.
• Sometimes several MCs get collapsed into one, sometimes the other way around.
• Since all the mathml is put on one line, some of the diffs are very difficult to parse. I wonder how difficult it would be to provide better formatting of the mathml. See, for one example, amsmath-01.
• I did not include any changes that are due to changes in luatex and luamml (see comments in Update some tests for texlive 2026 #1254).
• In at least one case an entire AssociatedFile disappears; see numerica-01. In another an entire Formula disappears; see longdivision-01.
• In some cases characters change, e.g. extarrows-01.
• In letterswitharrows-01, <mover> <mi mathvariant="normal">x</mi> <mo stretchy="false">→</mo> </mover> becomes <mi> <mglyph/> </mi> which definitely seems wrong.

[zauguin]

• The prime character ′ in mathml goes from <mo lspace="0" rspace="0">′</mo> to <mi mathvariant="normal">′</mi>. Same for !.

For ′ this is a luamml bug which is fixed in main. For ! this is a lua-unicode-math bug which will be fixed with the next release.

• Since all the mathml is put on one line, some of the diffs are very difficult to parse. I wonder how difficult it would be to provide better formatting of the mathml. See, for one example, amsmath-01.

show-pdf-tags just preserves whatever formatting the PDF attachment does or doesn't have and luamml can produce formatted output, but the whole process of writing it to a file and reading the file again strips formatting. I'm not sure if it's easier to preserve formatting there or to run something like xmltidy on the show-pdf-tags output.

• In letterswitharrows-01, <mover> <mi mathvariant="normal">x</mi> <mo stretchy="false">→</mo> </mover> becomes <mi> <mglyph/> </mi> which definitely seems wrong.

That's actually somewhat correct. letterswitharrows overwrites \vec when loaded with the vec-cev option with a non-tagging compatible version. Since unicode-math overwrites everything at \begin{document} it restores the compatible \vec, while lua-unicode-math keeps the overwritten one. Given that the point of that option is to get another \vec I would consider keeping it to be the correct behavior.