Describe the issue

Summary

The pgf/luatex tests have been flaky for a while on azure (see e.g. https://dev.azure.com/matplotlib/matplotlib/_build/results?buildId=17033). Based on the logs

E               ! LuaTeX error ...xmf-dist/tex/luatex/luaotfload/fontloader-2016-02-12.lua:11111
E               : attempt to get length of local 'lookupdata' (a number value).
E               <to be read again> 
E               \scan_stop: 
E               <*> .../work/1/s/lib/matplotlib/mpl-data/fonts/ttf/}]

I have traced this back to a piece of code in luaotfload that has been removed in early 2016 from luaotfload (latex3/luaotfload@0cf41df / contextgarden/context-mirror@7884878 are the specific commits, grep for local function ligature in src/fontloader/misc/fontloader-font-otn.lua (fontloader-2016-02-12.lua is a concatenated version of the lua sources)). I don't think it's worth spending more time on a 5y old bug(?) in luatex, so perhaps we could consider either updating the azure ubuntu images to 18.04, or, if we really want to keep the 16.04 images, figure out how to use a more recent texlive in it.