test/css: add some visually-verified test cases for our new features

test/css: add some visually-verified test cases for our new features

It's helpful to have some test cases for our CSS handling, but
unfortunately, it's not all that easy to write them. How should we
compare the actual output to the expected output?

For now we settle for a visual comparison. You can render the test
diagrams using the example program examples/svgtiny_display_x11, and
also open them in another SVG viewer. Do they look the same? Great.
The test cases should be simple enough that _subtle_ visual mismatches
are not possible.

Automating this is not an impossible problem: for example, we could
render the parsed diagram to a bitmap image using cairo and then
compare it to what we get using librsvg which is also cairo-based. But
that has downsides too (like the portability of librsvg) that we don't
want to address right now.