testing/sample_graphs: factor test sample graphs + add docs
Those graphs provide example of edge cases that are useful to test against in other modules as well.
This instance will be upgraded to Heptapod 1.3.3 (GitLab 16.9.6) on 2023-04-25 at 18:00 UTC+2
Those graphs provide example of edge cases that are useful to test against in other modules as well.