In the screenshot tests this works by setting SLINT_DEFAULT_FONT to
pointer to a directory, instead of a file. We then load all fonts in
that directory and consider their families the default unless a family
is specified. This way for "Noto Sans" a regular as well as an italic
version is registered in fontdb and returned in the list of font
fallback ids. embed_glyphs in the compiler then embeds those variants
and we find them at run-time.
