Add an endurance test

[bifurcation]

The script-based tests are designed to cover all of the functionality in the spec, but they do so "shallowly", e.g., using the smallest groups necessary to exercise the functionality. We should have deeper script test, but also the ability to run "endurance" tests that run a group through many epochs. The idea would be to create larger groups with more complicated trees to make sure that all the corner cases are covered correctly.