Fix race condition in PDF math rendering

[int-y1]

networkidle0 existed since v0.13.0.

Some background:

• MathJax didn't provide a way to tell if the math fonts have fully loaded.
  • For example, MathJax's typesetPromise creates the DOM elements without any knowledge of fonts.
• Puppeteer creates the PDF too early, so the math may not have a font yet.

[codecov-commenter]

Codecov Report

Merging #1810 (b65004d) into master (8e5b6b0) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1810   +/-   ##
=======================================
  Coverage   46.13%   46.13%           
=======================================
  Files         223      223           
  Lines       12698    12698           
=======================================
  Hits         5858     5858           
  Misses       6840     6840           

Impacted Files	Coverage Δ
judge/pdf_problems.py	37.66% <ø> (ø)

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