Position mprescripts at per MathML Core.
From [1]:
Place the mmultiscripts base and boxes at inline offsets
inline-offset and with their alphabetic baselines aligned with the
alphabetic baseline.
Also tweak subsup-6.html to compute bounding box of the
element at the time a test is run. Otherwise reflow due to resize of the
<div id="log"> element can make the test fail.
[1] https://w3c.github.io/mathml-core/#base-with-prescripts-and-postscripts
Differential Revision: https://phabricator.services.mozilla.com/D222614
bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1909417
gecko-commit: fabc61dc18efed097d9d499406d12c2094b75772
gecko-reviewers: emilio