bug: mathjax test is flaky

[simonhaenisch]

Pipeline executions on master randomly fail sometimes, because of the mathjax test where it fails to render the formula, either because the pdf is saved from the html doc before the mathjax script has finished running (i.e. replacing formulas) on the page, or because the script has ran before the formulas were added to the page.

Example failed execution: https://github.com/simonhaenisch/md-to-pdf/actions/runs/6558793122/job/17813089358

Maybe this is a hint for the solution: https://docs.mathjax.org/en/latest/advanced/typeset.html#typeset-math