Add Ahem font to allow list when installed by wptrunner.
This also renames the white list variables in the associated test files to allow
list. The pref name is not so straight forward to change.
Differential Revision: https://phabricator.services.mozilla.com/D207962
bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1892399
gecko-commit: c889e3972268157ead34b8a9af88bd8e95b2ea74
gecko-reviewers: jgraham