Add tests for rendering from in-flow children for MathML scripts, mpadded (#47351)
Currently, we already test that we render from in-flow children only for radicals [1] and fractions [2].
For more coverage, this adds analogous tests for scripts (msub, msup, msubsup, mover, munder, munderover, mmultiscripts) and mpadded.
[1] https://github.com/web-platform-tests/wpt/blob/ad7e196b12/mathml/presentation-markup/radicals/radical-rendering-from-in-flow.html
[2] https://github.com/web-platform-tests/wpt/blob/ad7e196b122827421b84dca83cda6d69e64eef41/mathml/presentation-markup/fractions/frac-rendering-from-in-flow.html