[Test Harness] Improve Graphviz related testing

[dduportal]

Following #46 discussion,

it appears that the current test for graphviz is too simple.

As explained by @mojavelinux:

We probably want a slightly more complex example to make sure we put some weight on graphviz to verify it works. Perhaps @pepijnve can advise. @pepijnve do you have a go-to example that you use (perhaps from the test suite) that thoroughly tests the graphviz setup?

This issue is to keep track of this task in the to-do list.

Hello @pepijnve! Could we have your inputs on this domain?

For everyone willing to contribute, do not hesitate to propose here any asciidoctor samples with graphviz that you feel could help.