[Gecko Bug 1788645] Do not reset font-weight/font-style when mathvariant is specified.

[moz-wptsync-bot]

In the past, mathvariant was cancelling the effect of legacy fontstyle/fontweight attributes by resetting the font-style/font-weight properties. These legacy attributes have been removed in bug 1783841, so remove this hack from Stylo and add corresponding WPT test.

Differential Revision: https://phabricator.services.mozilla.com/D156174

bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1788645 gecko-commit: 8161856de5f68141831645c0c4581b56c4e8ef32 gecko-reviewers: emilio