Write tests against static rendered png and svg outputs

[jayqi]

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.