Provide a user stylesheet with an `@font-face` rule for it.
Fix#6195.
Many previously-failing tests now pass, and a few previously-passing now fail.
Among the latter, `font-family-013.htm` and `fonts-013.htm` are testing
that the Ahem font is not used for characters it doesn’t have a glyph for.
They were passing because Ahem was not available at all,
and now fail because we don’t implement font fallback correctly.
The others also use Ahem, but I don’t understand yet what’s going on exactly.
