Write tests against static rendered png and svg outputs

Right now we don't fully check rendered content on diagrams, but we should.

~We should do this in a parameterized way to reduce the amount of testing code to be maintained. We can use pytest's fixtures to load the example classes and/or created diagram objects.~ Addressed in #21.

#21 addressed creating the static outputs, and has a test that checks the against the static DOT files. Still need tests that check png and svg formats.

Start by reading the test from #21 that checks static DOT files and inspect the example classes or diagram objects it uses. Use pytest fixtures or parameterization for the existing cases, extending coverage to PNG and SVG outputs. Done means rendered PNG and SVG content is checked alongside the DOT outputs.

Written by the indexing model from the issue text.