This test isn't generating any image output, so it's silly to
have a 60x60 reference image for that. Not only that, but the
rgb24 cases have always been failing due to a missing rgb24-
specific reference image, (but pdiff had been hiding that).
The test passes an empty string to cairo_show_text, cairo_text_path, and
cairo_text_extents, and NULL and an invalid pointer, with zero num_glyphs to
cairo_show_glyphs, cairo_glyph_path, and cairo_glyph_extents.