Add a few standard test DAGs

[ramfox]

Right now we are creating more or less the same DAG each time. Might be fun/a good idea to create a few DAGs (one with a single node, one that looks more like a list, one that has one parent with a bunch of children, one that is balanced, one or two that have a more organic shape) that are cached for testing purposes. Would make it easier to write tests and would remove the repeated creation of the same tree over and over again.