Fix mathml/presentation-markup/spaces/space-like-003.html
Make <mi> use the regular font so that Ahem applies and measuring it
is precise and not dependent on system fonts.
Differential Revision: https://phabricator.services.mozilla.com/D258188
bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1931193
gecko-commit: 7062ee63a571cf647041d9083d492def92476ce1
gecko-reviewers: fredw
