test_svg_reference: new macro to have reftests from two SVG files
test_svg_reference: new macro to have reftests from two SVG files

Instead of test_compare_render_output, which is slightly awkward
because one must insert the SVG document as a Rust string.

Part-of: <https://gitlab.gnome.org/GNOME/librsvg/-/merge_requests/616>

GNOME/librsvg,GNOME/librsvg,GNOME/librsvg,GNOME/librsvg