tests: Add visual test for colored underlines.

Note that the underline's vertical position is wrong, meaning that the
expected.png will have to be updated whenever this is fixed.

tests/tests/swfs/visual/text/underline_color/test.toml

@@ -0,0 +1,8 @@
+num_frames = 1
+
+[image_comparison]
+tolerance = 0
+max_outliers = 12
+
+[player_options]
+with_renderer = { optional = false, sample_count = 1 }