COLRv1 test font visual rendering test

Test passes if only color glyphs are seen below. Basic sanity check for variable COLRv1 rendering, additional testing is done in the Skia GM test for COLRv1, see Skia's gm/colrv1.cpp for details.

The following two rows must feature purple (first row) and green (second row) in the glyph gradients.